This seems one of the easier methods but still looks fairly time consuming. It requires a trip to Jo-Ann's or a craft shop for some supplies. Having an Exacto knife in the house is always a good idea.
Sorry I can't get the vid to embed but click on the link
How to Make a Cover for a Kindle
The problemish with it is most hardcovers have a thicker spine than a reader. If you used some batting underneath, that would compensate for the height differential. You can still just glue it together. I'm not sure I'd use Elmer's Glue--sorry. I'd probably use a dedicated heavy use craft glue. The benefit, of course, to using some batting (it's what is used in quilts and comes in long rolls, from which you can buy what you need. Buy 12" or something like that in the thinnest weight they stock.) is that it offers padding and protection for your reader.