I was contemplating this myself!
I'm thinking of getting some wool batting, rolling it up and covering it with some sheeting.
I also heard someone say the just stuck a pool noodle under the sheet! I think I'd still want some wool around it, just for comfort, y'know, but talk about "using your noodle" Ha ha

Okay, sorry, that was bad. Preggo brain makes me weird and goofy.