I use a couple of things depending upon what is available at the time
The absolute best was 75% teflon sheeting which is a foam. slightly harder than a thong rubber ( the ones under your feet for those with funny minds ) which I used to get scrap fom a local factory
Next was cotton insertion buytl rubber , but it too comes in a couple of densities and it must say "petrol safe " or something similar
Down here you can not use cork sheeting unless it is whole cork because the solvents that get put in out fuels ( which are nothing like petrol ) attack the glues used on ground cork sheeting even buytl cork sheeting and they crumble away after a time .
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.