Xmm, just on time....
Few weeks ago a friend of mine managed to figure locally two beater, non-functioning M3s, bought both for just a mere of 220$. I decided to pick one of them specifically for my 90mm Elmarit. Besides, it appeared to be excellent with my 50mm 'Cron, even though I wear obstacles. 90mm frame of M3 appears to be almost as convenient as 500mm frame in my M6 (yet a bit smaller, but still very nice one).
I can imagine, once they will be back from CAL/repair (hopefully will be reasonably priced), it will happily carry both 90mm and 50mm lenses, or at least will be glued to 90mm one....