Originally Posted by
roadcrankr
Is this doo-hickey a "claw?" Never heard it called that. You need one of these, anyway.
Yes, it's a claw, as in claw mount derailleur.
And many have the built in offset (like the one pictured) so the RD is in the same place it would be with a built in hanger.
The OP has plenty of options, and if this is to be only used on a trainer, he doesn't need to spend much.