How to mount a derailleur hanger on this frame?
You have answered your own question, correctly too. 

Those generic hangers are exactly what you want.  The golden piece held on with the short bolt should slide up the dropout and rest against the curved end, and your wheel axle clamps down on the hanger.

Then your rear derailleur will thread into the large 10mm hole in the hanger.

Its not complex, but it can be fiddly with tolerances.  Good work, all I can do is confirm you're on the right track.