I have cushions from an old couch, long story on why I still have them, but I do. But clothing or leftover carpet padding (Oooo! I have leftover carpet padding, that's what I am using next time!) or something similar will work I am sure. Turn the bag inside out to show off the white plain surface. Then cut your material to size if it's a cushion or pack it with your material until it's tight.
Then, fold up the top and secure close with duct tape and draw on whatever kind of target you want.
Then have at it. I had to go take it for a few shots to see how it went. I did poorly since I couldn't find the arrows that went with my bow. (I know excuses excuses, but got to brag on hubby, they were both hand made, I just didn't want to wield the 60 pound bow that went with the arrow I found that evening.), but you can see the holes. It's not exactly self healing, you will have to redo it in the future, but I think you will only have to replace the bag, the inside would be just fine to reuse.