The threading for rear derailleur attaching bolts is M10x1.0 so you need a hole in the adapter of that dimension. Older and cheap bikes use a "claw" rear derailleur mount which is a bracket the rear derailleur bolts to and is then clamped in place between the rear dropout and the rear axle fixing nut or qr skewer nut. You might be able to rig something along those lines or, better, get a commercial claw to adapt like this one:
https://www.amazon.com/SunRace-Shima.../dp/B001PT8M8C .