I have vent covers on all my vents includig the fan - installed by a previous owner. I can't get to my TM right now, but if my memory serves me, they are standard MaxxAir vents for RVs. The one on the fan is a little larger than the other two. They appear to be easy to install and do, in fact, let you keep vents open when it rains. In addition, if you happen to forget to close a vent before closing the TM, it protects the vent lid from blowing in the breeze as you tow!
You should be able to find them at Camping World or any online RV supply place.
http://www.campingworld.com/shopping...nt-covers/1529