04/16/2020:
Okay, larger update.
You're looking at probably the worst CAD mockup of anything ever and the sum of all my concentrated effort and skill.
Pretty excited about it, nevertheless. Plan is to start a print of a prototype (probably 5% infill, 1 mm thick walls) tonight and test fit tomorrow. If it works, I'll print a more substantial version (say, 10% infill 2 mm thick walls) for the road. This is the passenger side. The driver's side will just be a mirror (zing) image of this model.
The side with all the reliefs faces the interior of the recess left by the old side-view mirror mount. I'll be heat setting threaded inserts, mentioned earlier, to this face to create mounting points that are same as factory.
The tricky parts is going to be mounting the low-profile side mirror. The shape is not so straightforward as this mounting plate. You can see there's some curves and bends that would need some really good measurements and patience if I wanted to build a recess into the flat side in CAD.
The other option would be to router is out, which might be worth it to try on the prototype. I also thought I could maybe heat set it but I'm pretty sure that'll be too much heat for a piece of metal with that much paint on it.
There's factory weather stripping and foam that will hopefully make everything snug. I'd like to also cut out a thin piece of rubber to place underneath this plate to seal up the interior from road noise. Shouldn't be too hard.