Decided to add power/data jacks to the left and right downtubes. This provides the OnSpeed wiring for glareshield displays based on my huVVer-AVI devices.
These jacks use 4-wire audio cables, repurposed to provide power, ground, data-in, and data-out signals. This is identical to my Harmon Rocket panel jack that I use for my glareshield mounted display.
The advantage of using this audio cable is that it is compact and black-- easy to run forward of the downtubes and won't reflect in the windscreen. I bough two cables, and will cut them in half, so I can get both the plug and jack from one cable.
Now, I can 3D print whatever mounting I need to add instruments connected to the downtubes, or to the boot cowl. For example, a lighted compass, additional huVVer-AVI units, or an OnSpeed/huVVer-AVI for the copilot.