Anything that has the ereader well away from the corners (centered in the case) or corners protected and closes securely so that it doesn't flop open if dropped. Should be hard/firm as well, not soft, no accessories in the opposite side to press on the screen when closed (some have a pocket for the cord etc...not good in my opinion).
I use this Verso cover with mine (many other colours/models available):
http://www.lightwedge.com/Shop/Cover...s-demask-cover