In the rain, I use a heavy tarp, folded, open on each end. It forms an "A frame" tent over the generator. I secure it with rocks. The air intake and exhaust are open to the air but protected from the rain. The tarp is easy to deploy and to put away.
I am probably being too cautious, but why not? It is so easy to protect the generator with a tarp, as long as it gets enough air at both ends.