Back when Gen I's were the current model, a Forum member fabricated a bunch of cool items, including a mounting plate for the GXM-30 puck used by the Zumo 550. The mounting plate screws onto the top of the clutch reservoir with longer replacement screws, and the GXM-30 is mounted on the bracket with screws that go from the bottom of the mounting plate up into the bottom of the GXM-30. It is clean, and very secure.
That Forum member is no longer around, and I don't know of any equivalent mounting system for GXM-30s or the GXM-40's used with the Zumo 660.