Pending more test rides, the flashlight fairing mount works well:
Despite all my fussing with three rotational angles, simply tilting the mount upward by 20° with respect to the fairing clamp aims the flashlight straight ahead, with the ball nearly centered in the clamp:
That obviously depends on the handlebar angle and the fairing length (which affects the strut rotation), but it’s close enough to make me think a simpler mount will suffice: clamp the flashlight into a cylinder with a slight offset angle, maybe 2°, then mount the cylinder into a much thinner ring clamp at the 20° tilt. Rotating the cylinder would give you some aim-ability, minus the bulk of a ball mount.
Or dispense with the separate cylinder, build the entire mount at the (now known) aim angle, clamp the flashlight directly into the mount, then affix mount to fairing strut. Rapid prototyping FTW!
For now, it’s great riding weather …
The OpenSCAD source code as a GitHub Gist: