Options to repair rust in Duster fresh air vent

I've thought of going in from below - but it seems that I would only be plugging a leak and not addressing the long-term problem - rust on the top-side of the metal. Also, looking up from below, the rusted-thru area seems to be at the base of the round ring that sits at the top of the vent housing. To remove that housing does not look like a good experience since one bolt is behind the vent box (towards front left). If I get it off I don't know if I'll ever get it back. I suppose with enough persistance I could do that.

Seems that I need to kill the rust, seal the metal, and either add a screen below the vent or come up with a removable plate when I repair the upper firewall that will provide a clean-out hole from time to time. After I get the car painted - I may promote the car to a spot in the garage and let one of my newer cars deal with the falling leaves.