Thanks for the tips, guys! I was thinking about those 4x8 sheets of styrofoam insulation from Home Depot when I was there the other day, but I wonder how many inches it would take to stop an arrow? I don't know how dense that insulation is. It seems like 4 or 5 of those alternated with some cut up cardboard boxes would do the trick. I could put an old left-over piece of carpet on the very back.