I got this holder from REI for under $10 not including cage. I used it after stripping the paint and building up a '73 Super Course to test ride before painting. I had proper bosses added during frame restoration, and found a nice vintage cage on Ebay that I now use. I will keep the clamp on mount for longer rides. It is rotable 90° for frame or handlebar mounting. It comes with a rubber sleeve to protect the paint, and looks kind of wimpy, but it really pretty solid in use.

I know, in the out of focus close-up, the gear cable is under the clamp. Don't try this at home!