I know this is not what everyone wants to hear. On one of my old cars, before I had a welder I did cut the cowl vent panel off to access the hats. Both hats had completely rusted away. I taped the holes from underneath and laid 2 layers of fiberglass over it. I did this back in 2004 and it has held up fine, no leaks. While I was there I made the wash-out openings on the side larger to allow things like leaves to wash out easier. What I do miss, since I sealed the hat openings, I no longer get the fresh air, which sucks on a hot day. It was a factory heater delete car so no defroster either.
Funny thing, after I was done, I had a shop tack weld the cowl vent panel back on.