Hide Phone in a Hollowed Book (Felt Liner Insert)
A felt-lined hollow book can hold a powered-off phone for temporary storage if the finished cavity fits the phone without pressure and leaves the book stable. Measure the phone with its case, keep the spine intact, cut in shallow passes, and secure thin felt to the cavity. Never charge or operate a phone inside the closed book.
A neat insert depends more on careful measuring than on a clever cutting trick. The key challenge is making a cavity that looks tidy but still leaves enough page block and cover to hold their shape. I plan the finished space first, then choose the book and cutting method to suit it.
The measurements below are fit allowances for this project, not code rules. There is no building-code dimension or airflow rating for a portable book insert. Your phone maker’s safety instructions still apply.
Plan the phone cavity
A phone cavity is the clear space left after the felt is installed. Measure the device in the case you plan to store with it, then add room for easy removal. Check the lined cavity at the end; the book’s outside dimensions do not tell you how much usable space remains.
Use a ruler or calipers to measure the cased phone’s length, width, and thickness. Measure at the widest points, including raised camera parts, buttons, or case edges. Record the figures before marking the book.
Make the finished cavity at least 6 mm longer and 6 mm wider than the phone, and at least 3 mm deeper than its thickness. These are practical fit allowances, not universal standards. A soft or bulky case may need more space, so test the fit before finishing.
If you use 2 mm felt on both side walls, add 4 mm to the rough opening’s length and width. Felt on the cavity floor also reduces usable depth by about 2 mm. For example, a phone measuring 150 × 75 × 12 mm needs at least 156 × 81 × 15 mm of clear space after lining. With 2 mm felt on all sides, aim for a rough opening about 160 × 85 × 17 mm.
| Measurement | Minimum finished clear space | Add for 2 mm felt on opposing surfaces |
|---|---|---|
| Length | Phone length + 6 mm | Add 4 mm for two felt-lined ends |
| Width | Phone width + 6 mm | Add 4 mm for two felt-lined sides |
| Depth | Phone thickness + 3 mm | Add 2 mm if the floor is lined |
Do not compress the phone to make it fit. A snug fit can catch buttons or make removal difficult. Next step: write down both your target clear dimensions and your larger rough-cut dimensions.
Choose a suitable book
A suitable book has enough usable page-block thickness for the cavity while keeping its spine and outer cover intact. The remaining paper must still hold together as a block. If the opening would leave a weak shell or reach the spine, choose a thicker book instead.
Check the book before cutting. Open it and look for a loose cover, cracked hinge, or damaged spine. Measure the page block where the cavity will sit, not just the book’s overall thickness. The usable depth must allow the cavity, its felt floor, and a continuous layer of pages below it.
Plan a border of uncut pages around the opening. There is no single safe border size for every book: page condition, book size, and cover stiffness all matter. If the remaining border looks thin or fragile, reduce the cavity or pick a larger book. Do not cut into the spine or remove so much material that the block shifts or the cover bends.
A hard cover can help the book keep its shape, but it does not make a weak page shell strong. Next step: confirm that your marked opening fits within the page block and leaves the spine untouched.
Gather tools and prepare the pages
Preparation means securing the book and marking the cut so the cavity stays even. A steady setup helps prevent a slipped blade, ragged edges, or damage to the cover. Use hand tools you can control, and follow the safety directions that came with them.
You will need a sharp craft knife or utility knife, a metal ruler, a pencil, and a cutting mat or other firm sacrificial surface. Wear eye protection, keep your free hand clear of the blade’s path, and cut away from your body. Replace a dull blade rather than forcing it through the pages.
Mark a centered rectangle using the rough-cut dimensions. Centering can help preserve an even border, but do not shift the opening toward the spine just to make it look symmetrical. Confirm that the rectangle leaves the planned page border on every side.
Clamp or hold the book so its pages cannot slide, without crushing the cover. If the book cannot be secured steadily, pause and reset the setup. Next step: check the marks, cover clearance, and spine clearance once more before cutting.
Cut the hollow-book opening
The opening is formed by removing the marked pages in several shallow passes. Slow cuts give you more control than trying to slice through the whole block at once. Stop if the book shifts or the blade starts to wander.
- Put the book on a stable work surface with the cover protected from the blade.
- Follow the marked rectangle with light, controlled cuts. Use the ruler where it can be held safely.
- Remove thin layers of pages at a time. Keep the blade upright so the cavity walls do not slope sharply.
- Pause often to check depth and shape. Stop short of the planned finished depth, leaving room for the felt on the floor.
- Check the outside cover and the spine as you work. Stop before cutting either one or leaving a fragile layer of pages.
Do not use force to speed up the cut. If the blade binds, remove the tool, inspect the pages, and continue with shallower passes. Clear loose paper from the cavity as you go so it does not hide the bottom or catch the blade.
The cavity does not need machine-perfect walls, but it must be large enough for the lined phone and leave the book stable. Next step: test the opening with the phone still in its case before adding felt.
Fit the felt liner
A felt liner is a thin, soft layer bonded to the cavity walls and floor. It can cover rough paper edges and reduce direct contact with the page block. It also takes up space, so confirm the final clear dimensions after installation.
Choose thin felt, such as 2 mm material, and measure its thickness rather than relying only on a package label. Cut pieces to fit the walls and floor without bunching. Leave no loose flap where a phone corner could snag.
Apply an adhesive suitable for the felt and paper materials, following its label for use, ventilation, and curing time. Press the felt smoothly into place. Wipe away squeeze-out before it dries, and remove loose fibers. Do not put the phone into the cavity until the adhesive has cured as directed.
After curing, measure the lined cavity and compare it with the phone. Insert and remove the cased phone gently. It should move freely without pressure on the case, buttons, or camera area. If it catches, correct the felt or enlarge the cavity carefully; do not force the phone in. Next step: inspect the liner and fit before closing the book.
Verify the fit and use it safely
Verification is a final check that the phone fits, the book stays sound, and no loose material can snag the device. A felt-lined book is for temporary storage of a powered-off phone, not for charging or operating it.
Check each item before use:
- The cased phone fits without being squeezed and can be removed without pulling at buttons.
- The felt is secure, with no loose fibers or sharp paper edges.
- The cover closes without force, and the book remains stable when handled.
- The adhesive has cured according to its label, with no lingering odor that signals it needs more time.
- The phone is powered off before storage.
Keep the book closed only for storage of a powered-off, non-charging phone. There is no universal airflow or CFM requirement for this kind of portable insert, but that does not make it a safe charging enclosure. Paper and felt can limit heat loss. Remove the phone before use or charging, and do not add a battery pack, cable, or other heat-producing electronics.
Apple lists an ambient operating range of 0–35°C for iPhone. That range is not a guarantee that an enclosed or charging phone is safe. Follow the instructions for your own device.
Reinspect the insert after its first uses. Stop using it if you notice heat, a swollen battery, compressed felt, adhesive odor, or a distorted cover. Next step: store only a powered-off phone, and remove it if anything changes.
Worked fit examples
These examples show how to apply the allowances; they are not reports of tested products. Use your own phone and book measurements, since case size and book construction vary. The figures assume 2 mm felt on both walls and the floor.
| Example phone size with case | Minimum clear cavity | Approximate rough opening with felt |
|---|---|---|
| 150 × 75 × 12 mm | 156 × 81 × 15 mm | 160 × 85 × 17 mm |
| 160 × 78 × 10 mm | 166 × 84 × 13 mm | 170 × 88 × 15 mm |
For the first example, a book whose page block cannot provide 17 mm of cavity depth plus a stable layer beneath it is a poor choice. The needed lower layer depends on the book’s condition and build; do not cut until the remaining block looks sound. In the second example, if the lined cavity measures less than 166 × 84 × 13 mm, the phone does not meet the stated clearance. Next step: adjust the book choice or cavity plan, not the phone by force.
Frequently asked questions
These answers cover fit, finishing, and safe storage. The main rule is simple: the insert is for a powered-off phone only. Do not treat a paper-and-felt cavity as a charging case or a heat-managed enclosure.
How much larger should the cavity be than the phone?
Make the finished cavity at least 6 mm longer and wider, and 3 mm deeper than the cased phone. Add felt thickness to the rough opening.
Should I measure the phone with its case on?
Yes. Measure the phone in the exact case you plan to store with it, including raised edges and camera parts.
Can I charge a phone inside the book?
No. Remove the phone before charging. Paper and felt can restrict heat loss, and the insert is not a tested charging enclosure.
Can I leave the phone powered on inside?
No. Store it powered off. Remove it before use.
What felt thickness should I use?
Thin felt is easier to fit in a small cavity. Measure the material and account for its thickness on each lined surface.
How do I stop the phone catching on the liner?
Fit the felt smoothly, remove loose fibers, and test insertion and removal after the adhesive has cured. Do not force a snagging phone.
Can I cut into the spine to make more room?
No. Keep the spine intact. Choose a larger book or reduce the cavity if the planned opening would reach it.
What if the cover no longer closes flat?
Stop using the insert and inspect it. The cavity may be too large, the page block may have shifted, or the cover may have distorted.
(This article was written by one of our staff writers, Nathaniel Pierce. Visit our Meet the Team page.)
