The eventual counterweight, assuming lead, would be a cylinder 2.5" in diameter and ~3" long. If I can't find a source for such a weight, then I'll machine a container from aluminum and fill it with the shot. As such, the smaller the shot the better to pack in the space. I might try crushing it together with my press. I don't anticipate trying to melt and cast it.
I did find a lead manufacturer that makes sash weights and who might have what I need. But until I know the exact weight I can't source anything.