I also highly recommend a "cap" over every vent cover - rain sensor or not. The folks from Fantastic Fan hate covers over their fans because of the restrictions they found they cause. That's why they started making their own that doesn't ruin the performance of their fans. It's highly recommend to use only a Fantastic Cover over your Fantastic Fan.
The MaxxAir vent covers are nice but you can figure on reducing the air moving capability of your fan by more than 50% of use one of them. Use them over your other vents but never over the Fantastic Fan.