We borrowed this basic design from another group, and modified it to work with the materials we had on hand. We used scraps of 1/2" plywood. Mini-marshmallows make great ammunition. Holes were pre-drilled for the nails. Holes were also pre-drilled in wooden spoon. We tried toothpicks instead of nails, but they did not have the necessary strength. A twist tie fastens the rubberband to the spoon.
Our Cubs had a great time with this. They were supposed to sit and have a battle, but they were soon running around and firing on the fly.