Tim:
Use a rubber gasket under the plate, something about the thickness of an inner tube. You will obviously cut the gasket to conform to the outside dimensions of the plate, but when you cut the windshield slot, cut it slightly undersize to enable it to seal snugly around the windshield's mounting post. This will help keep water out. A dab of silicone wouldn't hurt, either, if you are very careful.
Bud
