Well, I took out the plastic drawers out of the drawer holders. I then stuffed them totally full of straw. The plastic part has some holes in the bottom, but is mostly solid. So, poop can heat this, and guinea pigs can bury themselves in their straw.  All 14 CAN fit inside if they want to, and it is very toasty in there.

I hope it works, as temps are dropping into the 20’s tonight.