So it turns out the Bontrager Blendr system Garmin low mount doesn't fit the longer (100x series) Edge cycling computers. Fixing these little problems is what 3D printers are best at, right..?
This model is based on the original dimensions of the factory mount, but adds 25mm to the stem to allow my Garmin 1000 to fit snugly. The actual mounting disc is made with thicker tabs for additional strength and support when the unit is locked in.
Due to the additional length, the mount tends to vibrate heavily when riding on bumpier surfaces. An earlier prototype printed at 50% infill broke after about 60km, so I would suggest printing this at 100%!
(Disclaimer: Road testing continues with the 100% version - the current design may yet prove too flimsy over extended rough distances.)
Update / 27-May-2019
After ~1000km the 100% printed mount snapped in two places at the mounting rim. This was after some terrible punishment over cobbles, poor roads and bike paths. The spool of PLA used seems to have become quite brittle, and I plan to print again when I get a new roll of black PLA+. Stay tuned for updates.