This is going to sound horrible, and I fought it for a long time before seeing she's not in any danger:
Plastic shopping bag bed.

My girl kept dragging a shopping bag out and laying on it. I was scared she would suffocate so I kept hiding them away. She kept finding them though. Eventually she had a little nest bed of them and has been sleeping on them this summer.

It must feel nice and cool to her. She's happy, so I'm happy.