I always use a quarter, but have used wiffle ball golf balls cut in half too. They work fine and super cheap.

I talked to a guy out west who was racing an older wave (95 or 96 with the old style base) and he was kicking the crap out of the brand new boats upwind and he swears it's becasue his mast didn't overrotate when sheeted down hard.
I got beat by my son upwind too when he was on an older boat with a new sail (mine is new too and my boat is a 2000) and he thought the same thing, that he had better sail shape and the boats were set up exactly the same (as I did it)
Anyway, have a good Xmas