Picked this up on ebay.... I was skeptical, but I thought what the heck, for the price.... Fits like it was made for the PRS300. You can turn it on and off, and perform all the normal functions. I hacked it with the russification software so it is even more convenient. Heavy plastic with a double zip closure, folded and snapped in place. I have no affiliation with the seller.
Waterproof Camera Phone Pouch Case Bag Swimming beach