I just did a little o-ring research (owning a '90 964 that will eventually need at least a re-seal if not a rebuild gives me a vested interest in seeing this issue solved).
theoringstore.com have a couple of interesting items. First is a
broad selection of Viton o-ring cross sections. I'm thinking a thicker ring with the correct ID could be made to work.
Another is these
Viton 'back-up rings'. While not metric, they may have a workable size. This would increase the height of the package some, and indeed one could use one inside and one outside the standard ring. I'd never seen these before, but like the idea.
Good luck, please keep us (me!) posted.