I've used a flattened round ball, it works ok, but leather works better. I use elk hide, the heavier parts seem to work best in all my flinters. I cut it so it is the shape of the jaw, then cut a small hole in the middle so when it's folded over the back of the flint, the cock screw can turn without touching the leather or flint. Sometimes I nap a nick in the back of the flint to get the best fit in front of the screw.