I was in Barnes and Nobles the other day looking for something to use for a cover for my PRS-350 and saw these leather photo wallets which make a nice, reasonably priced case. They come in black embossed, red crocodile, and black crocodile. I bought the black embossed one and it looks great with the reader.
The size is 4-3/4 x 6-3/4. So there is a little extra room on the top, bottom and right side. I don't mind as it provides extra protection in the event I drop the case. And the wallet is pretty stiff too. The inside is a sort of micro suede. The tab is magnetized and sticks to the front cover pretty well.
Total cost was about $16.00 ($16.95 for the wallet + tax - 10% discount).
To adapt the wallet, I first used small nippers to snip the stitching holding the vinyl photo sleeves in. Then I cut a black plastic folder to insert into the inside pocket and serve as a base to affix velcro. I put another velcro piece directly on the inside pocket near the opening edge. So the reader is anchored in 3 places. The black plastic insert is optional. You can just put all the velcro right on the inside cover if you want.
Here is a link
showing the 3 photo wallet styles they sell which fit the 350.
If you're considering buying one, first check the Barnes and Noble page at www.retailmenot.com
as they frequently have online discount coupon codes or links to printable in-store coupons which are valid for stuff like this (but not for Nooks).