Have any of you put covers on your fantastic fans? I was wondering if you could recommend a good one that still allows good airflow.

Are you sure you have a Fantastic Fan? We had MaxxAir fans and covers on both our 2012 and current 2016.
If you have the MaxxAir Fans, they sell covers that the attachment hardware bolts right up to holes in the top site fan housing, easy no drilling mounting.

Have any of you put covers on your fantastic fans? I was wondering if you could recommend a good one that still allows good airflow.

I'm also guessing your fans are Maxx Air, with maybe the toilet/bathroom being a Fantastic (mine was but changed to the MA), so get the Maxx Air covers.

Whatever you do...Don't install the cover the wrong direction...Face it toward the rear of the RV.

There are a number of covers on Amazon. Be sure to use the Redwood Link.

With this one, you have a "hang-over" (no pun intended). Some of the covers have no overhang. When the rain blows, you can end up with some minor water under the cap. As long as the lid is closed, no issue.

https://www.amazon.com/Maxxair-00-955002-Black-Fanmate-Hardware/dp/B010D1VJSC/ref=sr_1_9?ie=UTF8&qid=1522721315&sr=8-9&keywords=rv+fan+cover

This one, you'll have to insert your own screws. I would suggest adding butyl tape under the cap, screw down the cover, then use the self-leveling lap sealant over the screws.

https://www.amazon.com/Maxxair-00-955002-Black-Fanmate-Hardware/dp/B010D1VJSC/ref=sr_1_9?ie=UTF8&qid=1522721315&sr=8-9&keywords=rv+fan+cover

This is a less expensive cap but note that there is no overhang. It's not necessarily required, but if I were buying a cover, the above would seem better. We currently have this lower one on over our bathroom exhaust fan. When it rains, you don't get soaked but "some" rain has come in. Nothing detrimental, but water is one of the biggest risks to an RV. Why risk anything?

https://www.amazon.com/Maxxair-00-955002-Black-Fanmate-Hardware/dp/B010D1VJSC/ref=sr_1_9?ie=UTF8&qid=1522721315&sr=8-9&keywords=rv+fan+cover

Just a suggestion...

Yes. Sorry you are right. It is a MaxxAir fan.Thanks

We have the big MaxxAir covers that almost look like extra AC units. Work great and we've had th fan vent open in horrendous thunderstorms and never a drop inside. And as Michelle said, they just clip onto existing holes with pins.

We installed the same covers as Vaughan and have used them in torrential rain and also when travelling down the road. Real easy installation.

Thanks Piperguy. Do you have a link to find this one? Sounds nice

Andy and I both used the MaxxAir fanmate modl 900 which apparently is no longer produced . Bummer

Thanks for all your help. We did end up ordering the MaxxAir II.