You will want to cut a hole in the hull on the side of the hull to make the repair from the inside then glue the hole back in place. Cut the hole on the side of hull that faces inward.

Another thing would be to put a port hole near the area to work through but I’m not sure if one would fit or if you could reach the area.

Make sure you use West System Epoxy don’t bother making repairs in polyester resins and bondo they will not last or be strong. Look at the plate to see if it corroded away. Once it is fixed and beefed up from the inside then you can repair the outside.

Make sure you sand and scuff the area really good and get it clean so the epoxy will stick. You will want to flip the hull so you are laying the glass on a flat horizontal area and not overhead.

Here is an example of the type of repair. I used some bondo as a filler and it swelled and popped, I ground it out and used West System epoxy with 410 powder it will take a little more sanding because it is harder but it will not absorb moisture.

http://themanshed.net/archived-projects/g-cat-catamaran/?g2_page=3