Have you tried slowly rotating the rubber doughnut, I say that because the 550F I'm currently fitting those into. will fit in more places than it first appears, you think it's right and it looks right but go a little further round and it looks and fits better.
Been a while since I did some genuine 500 clocks as I got rid of mine on both my 500s, much prefer the larger clocks like the 500K2 got.