Fast Eddy sells the BC adapter but it has an O-ring at the seat. I'll let you know how it works.Convex mirrors are supposed to show less vibration. The part number NBCP-1042.
I've been told to put washers on the bottom and tighten the $hit out of them. Haven't tried that .... can't understand how that's going to help when the vibration is in the handle bars therefore migrates to the mirrors.