I always thought you could just sandwich foil together with a piece of paper in the middle and the contacts secured to each of the foil pieces that are not in contact with one another, but are instead separated by the paper.
The wires to the two leads coming from the two separate pieces of foil could go to the camera shutter switch.
When you shoot at the layered foil, the shot will make contact between the foil pieces and release the shutter. If it is dark enough and you have a fast enough flash, you should see the bullet, shot, or what have you, hitting the foil you have the camera aimed at.