I'm sorry if this is off-topic, because Sandra is asking specifically about a Fantastic Fan, but there's another solution...
The
MaxxFan doesn't need a rain sensor, because it works in the rain. It is essentially the same thing as the combination of MaxxAir cover and Fantastic Fan which Victor mentioned, but in one unit, which folds down when closed, and which does not violate the fan manufacturer's recommendations.
The MaxxFan is not available with the mount designed for thin roofs (like the Fantastic Fan option), so creative mounting is required, as described in various topics earlier in this forum.
I'm quite satisfied with our MaxxFan, including with its operation in the rain.
Morgan, your description of the cyclic thrashing of the rain-sensing Fantastic Fan is great.

It also makes technical sense - the whole idea seems flawed to me, too.