These things are about as easy to make as a TV dinner.
All you need to start is a reasonably sized hardcover. All of the local dollar stores here have them. I forget how much but its cheap.
So here's what else:
1.) A book but we already said that
2.) Something with a sharp edge
3.) Elmer's Wood Glue
4.) Small paint brush
Now don't go following some stupid ass hobby-chat recommendation to use simple paper glue, or worse yet, hobby glue. Fill a 35mm film cylinder 3/4's of the way then add a teaspoon of water to make the stuff absorb quicker and easier. You could very well use the wood glue straight from the bottle but it'll take longer to dry and perhaps a page or two or three won't get coated because Elmer's is relatively thick. Choice is yours, this isn't rocket science.
You can use an exacto-knife if you don't happen to have man tools around but that'll take more time. Draw a line a half inch in from the outside edge and all around the page you'll begin cutting through. Save a few pages to better foster the illusion, but at least ONE.
Then take one of your sharper knives and begin cutting along the lines. If you've some grunt, 10 or more pages at a time can be dispatched with every completed square but don't feel like a pussy if you've no recourse but the exacto thingamajig. Women invented scissors, and drugged out Renaissance artists came up with the exacto-type blade because they were too stoned to trust a real knife for the work, so there's precedence for fucking off.
The book you've selected for this surgery should be capable of withstanding an inch and a half of intrusion; unless you're just looking for a place to stow those false eyelashes that keep disappearing. The choice is yours.
After cutting as deep as you want, cover the pages you'll be keeping, along with the front cover of the book. Aluminum foil or irritatingly useless plastic wrap or even wax paper will do the trick.
Use the paint brush to coat the outside edges of the pages you've just made that bodacious ass hole in. Lay it on thick. This stuff is cheap so there's no excuse to frig the job by being chintzy. Slather the facing of the front page with glue, then turn over that one page you've kept just for this. Make sure its reasonable afixed.
Then place the book down flat and put a weight on it to make sure everything stays together long enough to become one badass block of wood wannabe. I just dumped an old tool box on top, waited an hour or so then continued.
Open the front cover along with the other pages you've protected with the foil/plastic-wrap/wax paper. As long as that page you've placed directly atop the one you've been sawing through for the past few hours is really set in stone, then cut through IT to give yourself a brandy new first page that doesn't bear the marks and mess of your workmanship.
After once again being capable of peering into the hidey-hole, heavily coat the
inside edges of all of those rendered page edges. Leave the cover open and if possible set aside the project to dry overnight.
Awaken to the glee of having your very own stash-thing, and if you're me, you place it on the table next to the front door. Inside* are at least a couple of mini-knives, not to be confused with a real knife but something for slashing through twine, cardboards, etc., a pen, the cell phone Lisa made you buy cuz traipsing through the frickin swamp without means of communication is just too dreadful to even imagine, a windup watch, and a set of your wife's extra car keys for when she locks hers inside the vehicle.
These things are such a snap I've made four of them the past few days and have Glock's and Ruger's and things impossible to identify on a blog the Feds could read on a whim.
Clicking the headline link will take you to a place that has pictures of some fairy trying to do the same thing. That's in case my instructions weren't intelligible. So go ahead. I'll avert my eyes.
*Kershaw Shallot; 3 and a half inches of CPM S110V steel
Taiwan made Benchmark Mini-Ruckus knockoff, 4 inches of 440 C
Spyderco Native; 3 inches of CPM S30V