Unless you've already purchased one, you might look at the meditation cushions (yoga) on Amazon. They come in a bunch of sizes and shapes and pre-filled with buckwheat hulls. That's where I got my bag for Open class about two years ago and it's been great. Mine came much too full but had a zipper where I could remove some until it got just right. After removal, the buckwheat feels like it kinda locks together and holds it's shape.
I was helping a Hunter class shooter that is considering switching over to Open. His bag is full of some kinda lightweight small chunks, seems like maybe closed cell foam. Anyway, the filling in that bag wouldn't build up and hold it's shape behind my rear end like the buckwheat does. I kept feeling like it was gonna kick me out the back and dump me over backwards, much prefer my buckwheat.Â