A folded comforter would probably work as filler too.
I only thought of that because my mini dachshund uses a king sized comforter folded in a wooden tranny case I made into a bed for him. He has three wubbies then to sleep under to make his little nest. He's a weird little dog but I love him stinky breath and all. I am going to have to bookmark that pattern. He might be getting a new bed cover for Christmas.