I've just got one of these Nitrile clutch case gaskets for the M20..It may be an improvement over the cork original, though I haven't fitted it yet...Ebay No. 353955683441 ...Ian
I'll try mine in an outer cover...It's worth noting if you have new cork gaskets that they are best stored flat...If hung from a nail in the workshop or similar they will stretch...A while back I found out the hard way and stretched five new ones...Ian