Printing went smoothly after two preliminary passes to work out the sizes and alignments; this is the second pass, which you can tell because the mirror shoulder has three supports instead of the two shown in the solid model:

Mirror mount parts on build plate

One view of the parts, with the mirror shaft in place:

Mirror mount partial assembly - top

Another view, showing the bottom of the Elevation Plate with the recessed nut:

Mirror mount parts partial assembly - bottom

Assembling the two glue joints required an overnight clamping:

Mirror mount - glued and clamped

Then a layer of double-stick foam tape affixes it firmly to the helmet:

Mirror mount - on helmet

It’s a bit too big and way ugly, but works pretty much as expected.

Two lengths of heatshrink tubing now lock the mirror shaft sections in place; they tended to rotate slightly under normal vibration.

The OpenSCAD code and model have a few modifications from this object. The next one won’t have the third section of mirror shaft, which makes the shoulder and Az Mount smaller, and the Az Mount is 1 mm closer to the El Body. That shaves a few millimeters off the whole thing.

The mirror clamp out there on the end is much too large and has too many fiddly parts. I think a little printed doodad would work, but that’s in the nature of fine tuning.