No rear mech hanger

SJS sell a derailleur hanger for bikes without one. It sits on the axle and as Nelson pointed out, has an extra bolt that goes further into the dropout to keep it from spinning around.

Don't think that the small bolt is really necessary.
It all works well without.
Once the wheel nuts are tight enough, the reach mech or the bracket will not move more than necessary.
Don't drill the frame as long as it is not really needed.