I made a pat-able baby bottom with a knee high stocking and baby fat pellets. They fit perfectly in the gathered bottom bodies and you can make them any size you want. I tied mine in the center with cheap dental floss because it’s very strong and doesn’t degrade. The best part is they won’t get flat over time like polyfil can.
I been doing this with poly pellets, but I place a layer of stuffing down first. It makes the bottom so pat-able! I love your idea with the fat pellets, those are much softer and the dental floss.