The Great stuff was a nice idea --I thought of that one too, but here's a better way . . .

Look in the yellow pages for foam insulation installers in your area. Usually, you can talk them into filling up a couple big cardboard boxes with the spray they waste at the end of each day when they "clean out the hose." I've been able to get them for free, but sometimes a few bucks will secure them for you. The stuff they use is very tough and I used mine for about 8 years and shot everything -- including BH's into it with no problems. It would've lasted even longer, but my wife had started doing daycare at our house and the little kids tore up my target!