I reseated the 3D printed adapter, drilled a hole in it, and used one of the two set screws to help attach it to the center motor shaft. I'm pretty happy with how it turned out, I don't think it'll go anywhere. Also, I'll get a longer setscrew tomorrow and I'll be really happy with it.

I just wanted to add a quick video here, first motor tests.

Running the drivetrain pulls about 6A @ 48V after the initial current spike.