There are vent fan covers made for this exact purpose, well that and so you can have the vent remain open in rain, especially useful with the rain sensor units that automatically close.
I installed one on the van for my fantastic fan, works great. You’ll see them used quite often on RV roofs. My install was a bit tricky, since I have a roof rack built around the fan, I couldn’t attach it to the fan shroud, but I just used zip ties to secure the vent cover to the rack over the fan, which gave the fans cover a bit more room to open anyway.
Beware, there are options to get cover units that come with this clever EZ clip for mounting, however the metal attachment clips tend to rattle, I used one on the TT we had and was annoying when camped in high winds, would be worse if we had to listen to that overhead while driving. Otherwise the standard mounting system is bolted on. You can also use lap sealant or VHB tape to secure the cover to your roof, just make sure the surfaces are cleaned before using tape, but that stuff works!
Try searching on Amazon for RV vent fan covers