I got an e-book a while back about some off-base archery projects (building a PVC bow, etc). One of them was building a target. The way this person had done it (I hope I remember this right) was to collect a bunch of scrap cardboard from work. He then laid these on top of one another (like in a ream of paper or a phone book) and cut them to equal length and width. When he had enough, he sandwiched them between two 2X10s and compress them with ratchet straps. I have actually not tried this, but it sounds like it could work. Might be pretty dirty with cardboard scraps, but wrapping a burlap sack around it might do the trick. Hope this helps. Be well.