Mike,
I was worried about that, as well. Read up on the Fan-Tastic, and found out that it will simply stop trying to open if there's any impediment to movement. Doesn't harm it to open, (and it does open most of the way,) up and into the Maxx-Air.
I have both Fan-Tastics covered with the Maxx-Airs, and the two other vents with the smaller Maxx-Vent.
So far, no rain's gotten in. Except for splatters that day the rain was going fairly horizontally, down here, and triggered the Rain Sensor over the kitchen!
Steve