My recommendation would be to fix the leak first BEFORE installing an outside cover.
I had a slight leak on my Casita’s fantastic fan and eliminated the leak by removing the original caulking and replacing it with silicone sealant. The removal process is tedious (similar to the step in Gordon’s video) since I removed/replaced the caulking on all four sides and, using a drywall taping knife, a slight bit in-between the fantastic fan’s flange and the fiberglass . I squeegeed the silicone as best I could into the gap between the fantastic fan’s flange and the fiberglass and gave it a liberal coating on the flange and about an inch onto the fiberglass.
Later on I added this MaxAir II vent cover
. This particular model has mounting brackets that you install onto the fantastic fan’s flange; no holes are required thru the trailer’s fiberglass. And, should bugs (somehow) get between the cover and the fantastic fan, you can always unclip it on one side, hinge it up and vacuum them out.
The benefit of the cover is that you can have the fan open/operating in the rain. However, I would not recommend that you leave the fantastic fan up when driving; the concern there is the stress that the fantastic fan’s cover's support arm would be subjected to by highway bounce….