I got a yoga mat on sale for $5. I am going to cut it into 3 pieces 22" square. It's dark blue and after I paint it white, it should show the hits pretty easily. There was a video online about this but I don't remember the details. The goal is a self healing target that I can re-spray white.
Since reactive targets are expensive and throwaway, this might work. Depends on the type of paint I guess.