Does anybody use the stem mount below? I know it replaces a 5mm steerer spacer, but is it possible to replace say the bottom spacer under the stem, and have the Garmin pocking out the bakc of the stem, so essentially sitting on your top tub butted up against your steerer? (I have a Garmin 520 so relatively shortish)?

Best “side” image I could find of the fixed mount, looks like even the 520 will hit the steerer/stem in the configuration you propose:

Maybe the hinged one will work, assuming you have enough space to tilt it backwards; still, the unit will be very knee-knockable in this position.

Just use the top cap replacement mount that they do.