Unless you specifically want it above the speedo, I would just get a RAM ball which replaces one of the Handlebar clamp bolts, a short arm, and the diamond or AMPS base to mount the Montana cradle on - it's very versatile, and having the GPS between your bars means it's better protected, less likely to vibrate over rough terrain, and easier to reach with your left hand.
Works as somewhere to hang your helmet too.
Jenny x