I also had the grey cap from a sedan on my SL. It didn't leak but also didn't look right. A new one is available from MB for way too much money. Ones from old cars looked pretty ratty and their cork seals wouldn't seal anything. I found a chrome, non-keyed non-vented cap that fits perfectly, looks fine and doesn't leak, at Pep Boys for seven bucks. It won't win any concours but looks right on the car. One more alternative solution to your problem.
1969 280SL, manual, MB TEX, green/cognac