If you hang heavy fabric (I use a sheet of fleece) about 2 inches in front of the back of your box instead of attached to the back, be it cardboard or wood, then you don't need clay or anything to absorb the energy. Only attach the fabric at the top so it can move. The BB's hit the fabric, dissipate energy rapidly, and then just fall to the bottom of the box. You can do it with an old blanket, etc.
I also put a small angled piece of cardboard at the front of the box bottom so it slopes to the front. Then a small hole in the corner lets the BB's drain down into a bucket for disposal.