You'll need something rigid- - - - -not Neoprene. Phenolic, Nylon, Teflon, etc. Even a brass washer would probably work as there's no need for insulation back there at all. A thicker washer can be sanded or draw-filed down for the proper fit .
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!