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Home made, copper water pipe inside flexible plastic wire wrap, hammered flat on ends. Screwed to seat mount screw on bottom, Plexiglas sandwiched on top with GPS mount and XM taped on.
Using 1/4" plexi glass I cut a shape using the rubber noise deadiner from the cup holder. I then used double sided molding tape (not the cheap stuff) and stuck it in place. Then I stick the suction from the GPS mount to the plexi glass in the cup holder. Works great, easily removable.
My Sony NavU has a pretty unique suction cup; however my dashboard is too porous to reliably hold the unit (and the Armour All gloss doesn't help). So, I stick the suction cup on the windshield. It's a bit of a stretch to push the buttons, but then you're not supposed to futz with those things while you're driving anyway.
Since I move mine from car to car I used the visor clip from my Valentine One (don't use it-have it hard wired under the rear-view mirror) drilled a small hole in the suction cup mount of the Garmin another in the clip and attached the two with a small nut & bolt and just slide it in the visor of the Corvette or any other car I'm using.