I have done fiberglas repairs on airplane radomes and something you can use on hollow walls for backing is honeycomb material or stiff foam.
www.starboardmarineinc.com/Home/materials
I use a router if it goes clear through and cut out the area, taper the edges, glue stiff foam to the inside and then use 2 part epoxy and layer fiberglas on, squegee out all air bubbles, sand smooth, fill with gelcoat and sand, repeat 3 times and polish down to 1500 grit, then spray paint to match. There are all kinds of sites on boat repair that give you videos. Remember 2 things, you can be allergic to fiberglas resin and when sanding, wear rubber gloves and some type of protective clothing, taped at the sleeves and at your neck. Stuff itches!