Hi,
I'm hoping that someone on the forum has seen/used the MoonCraft covers made for the Prius, that have a single hole in the center. This would seem to allow air flow out (to help cool brakes a bit?) and not add anything appreciable to the drag. I'm holding out the possibility that this could be a good modification for summer driving.
Now that I see them again, this would be a tough mod, I think...