these don't look like much, but they're actually well thought-out and bulletproof.
King Cage - Bicycle waterbottle cages handmade in Durango, CO
The King Cage Universal Support Bolt is a hose-clamp with a welded M5 stud and nylok nut.
I recently put cages on my '74 International frame with these - I used strips of leftover Brooks leather beneath the hose clamps, but you could use strips of innertube or helicopter tape.
They're bulletproof and rock solid. They can hang under downtube, etc.
the hose clamps are spring stainless and will conform.
Another product I've used in the past that will certainly conform is Klickfix -
Velofred has these
- the metal strap is a long U and begins as an elipse, anyway
I've tried some of the other stuff (Elite, VO straps), but these two are the best by far.