As some mentioned here..a piece about 3mm thick leather does give the best results

I am using a viton flat flange gasket ring and this is working well for many years already. Leather dries out in months and cork shrinks quickly is my experience. Really happy with viton and can recommend.
In NL we can buy these at technirub.nl at 6 or 7 euro a piece.

email (option): Patrick.vandertier at gmail.co