The cork tiles can crumble with repeated use as a gasket, a good rubber one would be a better seal, but if you insist on cork type, email your address and I will send a 'sample' peice of correct gasket material for you to try.
Let me know what ideal thickness is too as there are a few to choose from.
I got a new petrol cap gasket from Draganfly last year. It was from a different bike to the M20 but you would not have known it, it fitted just as good. Don't recall the exact price but it wasn't daft. I'd considered making one myself but the price dictated it wasn't worth messing about.
The easiest way would be to grind a sharp edge on a bit of pipe of the correct diameter and then use a smaller pipe to cut out the centre. I'd cut out the centre first as it would stop the leather from stretching too much. I'd hit it on the end grain of a piece of wood, if you were really clever you could weld a piece of plate over the end of the pair of the tubes (after getting the spacing right) and cut the whole thing in one operation.
I didn't realise the gasket fitted like that. I thought it only had a small hole. I had been making the wrong gasket and probably assembling incorrectly.