instead of a metal washer, try a faucet washer....
they dont effect the weight of the head, plus they "give" enough so that if you hit something with them, it doesn't destroy them..(at least not for a while)
I shoot mine like judo heads all the time, and they do a number on small game
I also grind the sharp points of my field tips down flat, then put em in a drill press, making a hollow point field tip, knocks down on the penetration factor if you hit a stump or tree
You can buy a whole pack of rubber faucet washers for about three bucks at the hardware store.