I have an old Summarit 50mm f/1.5. I have had quite a struggle getting a lens cap for it. I'd love to have an original Leica cap, but they're either insanely expensive (with no actual guarantee of fit), or just not there. I made a couple of attempts on eBay, but neither cap fit.
Anyway, long story short -- I went to a camera shop today with my problem, and the counter guy came up with a brilliant solution -- binocular ocular caps. He found a pair of thick, soft rubber ones that were joined, one side of which fit my Summarit perfectly. When I got home, I simply cut the pair apart and mounted my $3.00 lens cap.
If you are in need of a lens cap, and don't mind not having a "real" Leica cap, this is worth looking into.