I started riding brevets this year and at the end of the 400-km, my eyes couldn't focus on the numbers on the gps or even read street signs. It was due to the dust, dirt and grime accumulated over 19 hrs of riding.
Lots o research led me to Wiley X Brick sunglasses. I talked to my optometrist's staff and they that any frame marked "CE" can be fitted with prescription lenses. I'm Not sure why this is the case - I assume the manufacturer meets EU directives and for frames, it must include something about being able to replace lenses?
Anyway, these are nicely foam-sealed around the edges and that seal can be removed and the glasses worn without it. The lenses aren't too curved either...that's a problem for my Rx.
With my eye insurance, all it'll cost me is $75 for the frames. I should have them in time for the next brevet. So, finding a frame you like and having it filled with your Rx might be a good option also.