A couple of Christmasses back I made a bunch of secret compartment books, and now I've made an "undercover" (snrk) for my Sony PRS-T1.
An ordinary secret compartment recipe is to glue a section of the pages together (and to the back cover) then cut out the middle with a sharp knife and lots of patience, but this one's a bit more complicated.
First off, I glued two sections at first, a big one in the middle and a small one at the end, because the ereader is thinner than the book I used. Then I cut a hole backwards, starting from the back, in the middle section, as big as I needed to house the reader, which should always be a bit bigger than you anticipate... At some point, I start cutting a smaller section out, leaving a small ridge at the top of the page. You can vaguely see it in the picture, it's one of the things that holds the reader in place when you slide it in under the elastic band.
Glue the inside pages together at some point...
The elastic band ends are housed in two indentations I cut out with the knife, then nailed into the paper with tiny nails.
The finger/clutching hole at the bottom I put in last because I was dumb, but didn't really matter. Before that, I'd glued one of the free pages on top of the hole, and then cut out the middle (like with the usual procedure with hollow books). And because I had a hammer and nails, I nailed the top and bottom margins together, though it didn't help with the top... There's a gap from the paper warping from the watered-down glue. Oh well.