Heck Semi that ain't too awful bad. You may just want to get a piece of 1/4" flat stock and put a "V" notch on one end and the a 90* bend about 1/8" off the barrel. I bet that would put you right on the mark. (or at least all around it!) And if it worked you could epoxy it in place.
On the flip side, I couldn't imagine what it would be like to get smacked with one of those balls! Well over an ounce of lead...ouch. That would leave a mark and a half.