What is the best way to make air cannon/bazooka ammo out of spray foam?

Ok, I built an air cannon/ bazooka and am making ammo out of spray foam. It works well, but I need it to stay in shape.

I rip off a small chunk of foam and make it into a pellet, with the smooth outside at the front. The only problem is that the back doesn’t stay in the shape, and the thing goes flat. I need a quick and easy way to stick the back together when I fold it. Maybe a quick drying glue and I need it to be fairly strong.