How about cutting a tennis ball in half. Use a 6" or longer thin bolt and a couple of nuts to fasten the tennis ball piece to the end. Make an incision up both sides of the ball so it can tolerate a 2" barrel and give you a perfect fit.
If you want to, you can fasten the other piece near the end one to provide barrel stability.
If you dont want to cut a tennis ball, you can cut a bow-tie shape out of cardboard and use nuts to secure it to the center. It should fit snuggly inside the barrel. Make it long enough to stabilize the bolt in flight.
Now that it's on my mind, I think I'll make some.
Jeff
