I've put some rice into old stocking legs and tied knots along the way to keep it even, then sewed in one lot across the bottom of the cot blanket cover, and will put another 2 across her chest area.
The rice shouldn't escape then. She is in a big bed but the cot cover is just the right size to get all the right places.

Geez, they are expensive online! It didn't take much to make this one up, thank goodness.

Her ears are very, very sensitive too, and it seems she can't lie still horizontally - I hope this helps, I'm starting the new bedtime routine this week.