While rolling out the fiberglass before glassing the deck, I ended up cracking a section on the back. The break was along a glue line.
To fix it I just glued it back together. First I put tape on either side of the break to keep from messing up all my sanding. I then injected some glue into the crack. I used green tape to pull the joint tight and reached in underneath to press the seam smooth and flush.
I was in a little bit of a rush so I heated the broken area with a heat gun to accelerate the drying. When it was set up fairly solid, I pealed the tape and sanded off the little bit of glue with 120 grit sand paper.