Get out a pair of scissors and a empty cereal box. Cut into one half inch wide strips. Then cut the strips into half inch squares. Drop one on top of the shot, just before closing. It covers the center of the crimp perfectly. From corner to corner, they measure about 3/4", which fits into the hull. They self center. You can do the same with a bit of Tyvek building material, used to wrap houses. Scraps are easy to pick up at any new house construction site. The overshot card does not need to be round, it just needs to be centered when you crimp the hull closed. I can make a hundred square cards in a couple minutes.
