I have used crushed walnut shells, and it does give you some weight. You can get crushed walnut shells at the pet store in the reptile department and of course Amazon. I was wanting to make a rather large one, say 8 x 8 inches and was wondering about steel wool.? I don't want to use the walnut shells because it would be too heavy and don't want to use a polyfil. I am in the south so humidity would be high.