One material you can use is Buna-n it will stand up to gas and oil.
You can get sheet stock from McMaster Carr on line
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www.mcmaster.com]
I can’t remember what the original was, it may have been a type of Phenolic, it was hard but that may have been caused from age. It needs to be fairly hard to with stand the fuel pressure. Cork won’t do.
I remember making a temporary one from Aluminum flashing.
Rolly