So did you find the 3M 4200 glue was able to hold the vent cap to the rod? That has been a concern for me–finding a glue that will work with plastic and metal both.
In my opinion, that particular vent is meant for regular RVs and mounted fairly high up away from splashing water, so the rain baffle would work pretty well if so. The vent was not designed to open and close–rather the cap is to be screwed down tight on the bolt, which is long enough to keep it from closing. I had to put in a shorter bolt to allow it to close down tight enough to seal on the weather stripping–but I like your idea better, IF the glue holding the cap to the bolt does the job.