I went to a flooring dealer friend and got an old piece of jute carpet about 20'x5' ( or whatever size) from their dumpster. Cut a piece 8'x5', fold over for a 4'x5' and sew along the 2 sides to create a "pillow". Fill from the open end with shrink wrap, plastic bags, etc. and sew the top shut. Should be about a foot thick.
This I use for a backstop and hang a bag target in front. Bag target is made from an old feed bag (dairy farmer up the road), filled with shrink wrap, old plastic bags and sewn shut.
All sewing is done with heaviest thread I have and a large needle. I build a 2x4 frame for the carpet bag and hung between 2 posts.
Unfortunately, for me, it's not as hard to miss that big a backstop as I would have thought...