i just got back from a 6700 mile road trip on my roadie, and prob 2k of that was in rain. i used my garmin nuvi 650 and with the MS batwin it didn't get wet other then some drops on screen.

What i did for a mount was cut a piece of sheet alum in a T shape.

how i mounted it was i bolted the sheet metal to the upper tree via the 2 bolts holes the and took my suction cup mount i never use, bolted that to the skinny part of the T (which was tall enough to clear triple tree upper handle bar mount). For wiring, all i did was solder a external accessory plug in the headlight buck on the switched 12v wire, used the car charger cable for the garmin and just taped it the accessory plug to make sure it didn't pop out, ran the wire up thru the opening in the back of head light to the mount/cradle. When i stopped somewhere, all i would do it pop it off the mount and put it in my saddle bag since cradle and wire where perm, this made it easy.

It sits dead center above my bar clamp, painted the sheet metal black, and its hard to even see it. 15 min job max.