before I knew such a thing as "rivnuts" existed, I decided an old French frame just HAD to include waterbottle mounts, and also chanced upon a basket-case trashed Mtn. Bke frame discarded on the street.
So...with more confidence than sense I cut out the MTB factory mounts, shaped the remaining frame tubing into "diamonds", then drilled the frame tubes in French frame and brazed them on.
It turned out so nicely I was sure I was a natural for brazing and soon discovered the truth: NOT a natural, just had some unnatural good luck, like beginners do.
So, kids, don't try this approach at home!
I would also be a little leery of rivnuts, have had mixed results with them. So maybe do a practice run on a trashed MTB frame, or a piece of one.