Just went through the rain cover process again.
When we bought our Beaver in 2003 there was only one vent cover. The concept of keeping rain out with an open FantasticFan vent was invented by Maxxair. They made one and it was white. I bought two.
The Monterey was equipped with electrically opened Fantastic Fans. They open automagically until they find mechanical resistance. Stock, the resistance comes at the end of the supplied arm. With the Maxxair covers, the resistance came when the Fan lid arrived at the Maxxair cover. It worked fine for the entire 12 years.
The original Maxxair vents were a PITA when it came to cleaning. Small nuts, bolts and washers had to be dealt with to remove the cover. Since the original there are now more choices from FantasticFan themselves, Maxair and Camco. On our (now gone) PleasureWay we used a FantasticFan vent cover. The lid opened all the way and simple pins allowed removal for cleaning.
Our newest rig has three FantasticFans! This time, due to color choices, a hinged design and larger vent space we picked the Maxxair II in "smoke". The lid does hit the vent about a 1/2 inch before wide open.
Regarding Jim's approach with blocks and calk, I wouldn't do that unless cleaning is determined to be done only by your spouse. Access for cleaning the cover and the fan would be worse than installing them!
