5mm rivnuts. Purist might not like drilling their frames to install them, but you can install them anywhere as bottle bosses and rack mounting points. Installing them doesn’t damage surrounding paint (other than the hole you drill for them). Metal clamps can also do a number on the paint, but I have to admit they maintain a vintage look as a vintage solution to a vintage problem.

The only place I haven’t been able to use rivnuts is on the skinny top tubes on my mixte tandem. In that case I used the bottle cage mounts that came with cheapo Bell water bottle/cage combos sold in department stores. They have a Velcro strap mounting system (like the ones used to hold mini pumps in their bottle-cage mounts) that I can wrap around both of the top tubes. The bottle cage mounting holes are standard so you can attach your favorite cage to them.