A waterproof auto body filler? There are some excellent ones out there that are fiberglass reinforced and are waterproof. They are the two part epoxy type, and run about $7-9 a quart. I have used it to repair the corners or rotted parts of trim after chiseling out the bad with good success. Your area may be too big for this, but its just a thought.
Good Luck!
Dodgeman