I have a hard plastic pretty basic black mast shim that works but is not a snug fit for my x9 160 boom a few years old. Yesterday I got my mast stuck (x9 430 rdm similar age) and had to do the two boom thing, but mine kept slipping under that sort of load, but doesn't slip windsurfing. Eventually got it to stay after mucking around for too long into beer time.
I used to have a rubber one which was a softer plastic and slightly fatter and not sure of brand, it worked great but split after a season. Any suggestions on a good one that makes a more snug fit with the x9?