Were your pre-made plates pre-threaded ba? Or had nuts welded to them perhaps? Got pics to post up here?
Seems like nut-serts would be a minimum. But I always like the idea of reinforcement, so making some nut-plates, if you don't want to buy the plates like ba did, would be a good move.
The lower mount looks like it's on a relatively flat surface, but the top mount might need to be just a narrow piece of metal to avoid the curve.
Do you have any pictures of those mirrors installed? Looks like the holes were already there maybe? Or did you just now get them laid out?
Paul