it's not you, it's crappy construction. You have a few ways to deal with it.

1-If the frame's surface there is truly flat, consider laying a sheet of emory cloth on a table and rubbing the hanger back and forth to sand it's mating surface flat.

2-If the frame's surface isn't flat, bed the hanger in body filler, and allow it to cure before installing the wheel.

3- leave it as is and tighten it as best you can.

No matter how you deal with it, you still should always check the hanger alignment with a wheel in and tightened. Actually, other than tightening it to riding standard, you don't have a choice because you need the wheel as the reference for the alignment gauge.

BTW- before doing this, try to figure how to get it so it's inner surface lines up with the inside of the dropout. You need to establish this position because the RD's travel range is designed around the assumption that it's mounted 6-8mm outside of the end of the right axle face (thickness of dropouts).