Any ole fleece you'll find at a fabric store should do the trick, I'd look for a thinner sort out of preference (to not add extra bulk to the dipe). It comes in several different names, it's all the same stuff though. The waterproof sort of fleece won't be sold in a regular fabric store, so no worries of buying it by accident.
Fleece does not fray, no sewing required.
The size and shape is all your personal preference. When we used fleece liners I cut rectangles out just for simplicity's sake.
Fleece does not fray, no sewing required.
