The parts are aluminum, so not hard to work with. I did the majority of the work by hand. For the piece that goes on the steering wheel, I trimmed it carefully with a Dremel, then straightened with a file, and finished by sanding across a piece of sandpaper on the workbench. Took some elbow grease to get like the picture, but not particularly difficult. The amount to take off is as you described. It's obvious how much to take off once the other side is on the adapter. The release won't go together until the right amount is removed.
The piece that goes on the adapter is a bit harder. I described making a fixture and trimming with a router. But that didn't work all that well, and if I were doing it again wouldn't bother. I mainly removed all the material with a Dremel. The flat surface is already there, so just need to work down to that. I didn't take the screws out that you mention. That would have released the locking spring assembly, and since there were cautions about how it should be adjusted, I chose not to disturb it. Not sure that would have made it any easier anyway.
If you're not comfortable with all of this, I would think a machinist could do it pretty easily. Probably with a milling machine, lathe, or both.