I remember reading threads on these, but never bought one because I couldn't try them out first.
However, I saw one in the boating section of a local sports shop and I took our readers in and found it fit my wife's PRS-505 pretty good. so I bought one for her.
I liked it after I got to try it out, and while they didn't carry one that fit my PRS-350 in stock, I was able to order a smaller one from the manufacturer that fit perfectly.
I am very happy with these for relaxing in the tub with a book.