OP; I would throw some votes in for the Riv-Nuts. I have used them a lot to add mounts onto car frames to hold fuel pumps and the like. Always seemed to work well.

Another option I have used on bike frames when I didn't want to torch up the paint; After removing the BB, drill the two holes for each cage in the needed location. Feed a length of button thread or fabric fishing line down each of the holes until the thread drops out of the BB. Tie the thread tightly to an M5 bolt, about 2 threads from the end, and pull it slowly back up and through the hole. Get a grip on the bolt with a pair of needle nose pliers, remove the thread, and put on a small diameter washer followed by a M5 nut. Hold the bolt with the pliers and spin down the nut, tightening it with an open-end wrench. I have most times dabbed a bit of epoxy under the washer to seal it up and a bit above to lock in the nut. When both are finished, add the bottle cage followed by another set of washers and a pair of nuts (cap nuts if available), but no epoxy there. I have bikes with this method that have lasted 25+ years without incident. It is hard to tell they are not regular braze-ons.

Hope that helps
/K