I looked around, I didnt see anything for the nook. You might be able to adapt a small netbook or dvd player case.
If you're looking to do it yourself, I would just reinforce the box you've been using with homemade paper micarta. Its basically just soaking or brushing a slow setting epoxy onto paper or thin cardboard, then applying the strips to what you want to reinforce (I'd also soak some into the original nook box). Its sort of a 21 century paper mache. I've used it to make repairs on stuff that needs durability and its very strong and tough, more like fiberglass or strong wood then plastic. I'd use glaze coat or Devcon 30 min as a second choice. You might want to do a test run on a different box to figure out how to deal with problems you'll have, like the box sagging in areas before dry, excess epoxy, ect.