I do suspect that if you write Garmin a letter explaining what you need, and enclose a self addressed stamped envelope, that will be the most trouble free way to get the part you need. I doubt they would bother charging you for that part if they don't need to go to the trouble of generating a shipping order, packaging it, etc.
FWIW, when I used a SP III on my moto, I removed that rubber cover (by removing the tiny Phillips screw that retains it) and stored it. The cover just got in the way, and it is not needed for waterproofing purposes. I figured if I left it in place, I would likely wind up tearing it off by accident. After 2 years and 70,000 km of use, I never had any water problems.
PanEuropean
