After a lot of searching, I found this clip. Totally worthless to me since it can rotate. I searched McMaster Carr and Grainger under "clip", "mount", "fastener", "quick release", "slide", "rail", and "bayonet". Nothing quite right, but a couple sheet metal things that aren't tight enough to do the job.

Anyhow, I think I could just leave a couple pan head bolts exposed (slightly above the surface and painted black), and then make a plate shaped like an E, but rotated to have the fingers of the E pointing down. The spaces in the E would slide under the exposed bolt heads. Pretty easy to whip up. I hope that's at all clear. I do have a dovetail cutter, but that'd require thicker stock and more work.

No need to create that mount. It already exists, and could work. Remember wall-mounted telephones?

https://www.summitsource.com/images/products/AC253I.jpg

I'll check some of them out. I can use the female side maybe. With coupling nuts I can attach the protruding bolts right to the car's metal structure, which is cool.

Figure I should determine the mirror before designing the mount. I was thinking bicycle mirrors because they're light, cheap, and compact to stow in the trunk. Also, I think they'll be easier to work with than going with ones from a junkyard. Should've looked for some when I was at Walmart.

Go back to Walmart. They have a mirror that should work and it's cheap.

When I was there I couldn't find any car mirrors and when I asked they told me they only carried blind spot mirrors now.

Oh, and interior mirrors (rear view and the like).

Just won two bicycle mirrors for $2 on Ebay. They don't have the handle bar mounting hardware, but that's irrelevant.

https://www.fuelly.com/attachments/fo...aaf0d47dfe.jpg