Weepy Electra Primary side filler cap

Two years in and I have just about got my Electra oil tight apart from a stubbornly little weep from the primary chain case oil filler cap.   Originally these had a thick red fibre washer which did a good job sealing the filler cap once it was screwed up tight.   Despite lengthy searches on line and at my local suppliers these seem to be no longer available.  However, the NOC spares offer an alternative large black rubber O ring which should work.   Sadly try as I might I can't get these to seal effectively because they distort when the filler cap is screwed up tight enough to stay in place.    This results in a very fine film of oil weeping down the primary case onto the exhaust after about 20 miles.    Needless to say I have checked the gearbox and clutch oil seals and confirmed they aren't leaking.
In desperation today I cut out two washers from a sheet of thin gasket paper and used them to replace the O ring.   30 brisk miles later and the weeping oil has disappeared!   
In sum has anyone experienced this problem using these rubber O rings?   Am I missing a trick in fitting it?  Does anyone know of a source for the original red fibre washers the right size?   I am going to order some thicker gasket paper to make a more permanent sealing washer but I would prefer to use an original washer.
To lift the mood I've attached a picture of the Electra after this evening's run - side by side with my dear old 1955 Ford Popular.   The perfect combination of classics! 
Nick