Balled up old clothes, bedsheets, blankets, towels, curtains - they'll stop a freight train. That's what I have my target stuffed with. My muzzy practice heads practically fall right out.

You can stuff the materialsin a cardboard box, in a burlap bag, in a duffel bag, in a piece of luggage, anything will work.

Another thing that will stop a truck - surprisingly - is balled up shrink wrap. As noted earlier, I've heard that plastic grocery bags will work too.