I had the same problems with petrol washing over the top of the tank on both a flat tank model 18 Norton and an early KTT Velo.
The complete answer is to contact Simon Grigson who is a Norton man. He has a choice of 2 materials, I seem to remember that one is neoprene (ok with oil but not petrol), the other is Viton (fantastic & utterly resistant to petrol but expensive).
Over the last few years he has supplied me the two washers. The result was so successful that I had to drill a bigger breather hole in the model 18’s tank cap!
Simon Grigson’s phone no. is 01256 381560.