i've just done mine as i had the same problem, i took the whole dash out and welded a plate over from the inside of the car and i also welded the battery tray back up, i do have some pics of this on my mobile but i need to find my data cable to upload them, definately do not rivet a plate on, as it won't stop the corrosion eating away (drilling holes for rivets could turn to rust spots over time), grind the rust away, seal the clean edges with primer and weld new plates, it is a lot of messing about but its well worth it.
Cheers Tom