Look deep into nature, and then you will understand everything better.
I bought two outsized O-rings from my local hardware store for a few pennies. I have trimmed them to size and glued them with a drop of super-glue. Not ideal for maintaining contact with oil, I guess, but 'twill have to do for now. I'll smear them with a bit of Vaseline before I fit them, whoopee!
I had to modify my original idea as the larger O-ring was too thick and the whole filter would not go together. I cut a substitute from thick gasket paper and fitted that instead. The original, flat rubber gasket is still there but compressed a little a allowing seepage. The paper one should fill that gap. I topped it up with about half a litre of oil (up to the red line visible in the top picture) and reassembled it. All looks good and no obvious leaks are apparant. I decided not to bother repainting it as it won't be seen by many people and I kinda like the slightly battered look that it now carries with 41 years of wear and tear. (Update, March 2014 - I changed my mind and gave it a lick of paint, click here to see )
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