A suggestion...

Make a cardboard cutout of each of the GPS you're investigating -- actual
size. Then take them with you next time you fly. After you preflight,
sit in the cockpit and place the cutouts in various places until you
find the most convenient & comfy place for you.

If you discover the best place is in the middle of the cockpit, in
thin air, then you know you need to figure out what type of
"arm attachment" you'll need.