Originally Posted by
FBinNY
Most low end derailleurs come with claws attached fir this kind of frame. They should be good enough for trainer use.
OTOH, if you already have a derailleur, or want something nicer search "derailleur mounting claw" and you'll find exactly what you need for a few bucks.
Can he find a low end derailleur with claw that will span 11 cogs? That's a lot of reach. Edit: a claw will mean that derailleur needs to reach in addition the the thickness s of the hanger to shift to the biggest cog.
A way to do this would be to get a second claw. Cut both to make a top that extends 1/2" below the dropout and a bottom that stops just below the dropout. Clamp them together to form a single hanger that now hangs where an integral hanger would be. Drill the overlap for 2 bolts. Not simple. The inside would need to be clean, so perhaps countersunk for the bolt heads. The attachment has to be rock solid. Failure here will be very expensive. Locktite red + lock washers.