i live in southern cali . . . not to much water here. And i would think water would get in at the corners of your design as well. If there was an indent where the vents or louvers are i would think it would help drain the water. Just a thought, not trying to knock you design or anything. Just thought i would through something that i saw out there.
And you would only see the opening of the vents from the rear. When your driving would that suck the air out?