While Joey was sleeping, I thought I'd try this out. I had some scrap fabric I got from Joann's (from one of those bins). I didn't follow it exactly, and the edges could be a bit tidier, but it works (and it motivated the girls to clean up their mess of play dough).
I used some batting as the mattress filler, since the poly fill stuff made it a little bulky. I sewed the blanket all the way around, so now it is like a sleeping bag.
The girls love it!