Here's a couple that I have done.

Take a 100# feed bag, and fill it with old clothes, pack it tight, and tie it closed. (works good)

If you live near a boat building company, have them fill up whatever sized box you want with foam. I have a BIG box full of the spray foam, and it works great, and lasts a long time.

Also, if you keep your eyes open, you can get rolls of the foam just like what is used to make the Block, and the Black Hole. Log home builders use alot of the stuff. Usually you can pick up a partial used roll for a few bucks, and them make your own Block target.

And, if you live near a window company, you can fill a large bag with nylon screen scraps. Pack it tight. You will be able to remove your arrows with 2 fingers.

Hope this helps you out.

Kip