Go to

http://www.POR15.com

and look at their kits. Short of welding new steel, they're probably the best bet.

The other issue is how water got to that point. The location you describe, just at the rear seat footwell heat duct means water is likely getting into the ductwork under the dash and following the path to the rear floor. Remove the windshield wipers and cowling at the base of the windshield. There is a plastic deflector that is supposed to keep water from draining directly into the cabin blower. The deflector comes unglued and droops, directing water directly into the ductwork.

The other likely suspect is the driver's footwell vent cover behind the left kickpanel. It's seal gives out and water directed into this area by the cowl runs into the driver's side compartment, front and back seat footwell.

I was lucky. I only had a 1" X 1/2" hole at my left heel. Using a heatgun and scraper the glued down sound insulation came right up and I painted the floorpan with rust converter then used marine putty on the inside and underside to plug the hole.

Good luck,
--
Erwin in Memphis, '88 745t 190,000 miles, '95 855t 92,000 miles