The Hopkins Flex-Mount controller came, and I got to work installing it.
Here's the package:


There's a Black Box (2x4x1-1/4), a display module (1-1/2 cube) and a control head (1x1x2) inside. Dimensions are approximate.

The Black Box can be mounted anywhere, but Hopkins recommends either the kick panel or the center console, not on the dash like other brake controllers. They don't really describe the mounting criteria explicitly, but the pictures strongly imply a vertical mounting surface, with the 4" length of the box pointing front-to-back and the 2" width goes up/down.
They don't call it a kick panel for nothing, so I ruled that out. The driver side of the center console looked like it might worry my knee. I settled on the passenger side of the console:

Since I hardly ever have a passenger, this should be OK. I did have to add about 6" to the brake switch wire so it could reach. I also made a small wooden wedge to get things vertical and give me a place to screw the top holes into.
The control head fit nicely on the lower dash panel, left of the steering column:

Looking out the windshield, you can see the display module in the lower left corner of the windshield. It's tiny, so look hard.

I haven't finished the wiring yet. The blue 12ga wire doesn't get here until Friday. Sometimes it really pays to buy locally.

When I finish up, I'll post in more detail on my build thread.