I was told that blue foam that's used in fish tank filters is the best thing to use as fill inside of carts. I got some at my petsmart, but when I cut a piece to put in a cart, I had a very rough time getting it to stuff in right. What's the best way to go about doing this? The foam has a blue layer and fades into a white layer at the bottom. Do I need to stuff with both layers, or just with the blue layer? When I tried using both layers, it was wayyy too thick and I couldn't get it to stuff in correctly. Any help would be appreciated 