I went to Joann's and bought a twin sized pressure relieving mattress overlay-aka an "eggcrate" mattress. The pointy foam looks like an egg carton. My mother had one on her bed and loved it, and the orthopedic dog bed we have has one, only it's supported by a thick layer of foam beneath it.
I'm using a fleece remnant I found at Joann's. I had it in my fabric stash. I love to pile through the remnant racks whenever I go in there for whatever unusual fabric I find. I don't know how much there was on the piece, but I think it was at least 3/4 of a yard. That's where I've stopped. All I have to do is sew up two sides and either put a zipper in, or put velcro/other type of fasteners on the remaining side. I've been doing several shows so I've been crocheting and knitting up a storm.