Here is a simple way to mount a GPS using a RAM mount.
Wherever you position your screen there will be a M6 tapped hole free on the screen mounting bracket.
Take a piece of aluminium bar and turn to approx 32mm diameter. Cut off approx 35mm long piece.
Centre drill with M5 drill and tap hole with M6 tap. Screw and threadlock one end of a piece of M6 stud into the aluminium and screw the other end into the screen mounting bracket. For added security screw a nylock nut on to the protruding end. I've tested this for 2000 miles with Garmin Zumo and vibration is minimal.
See photos.
John G