Routing the center plug, I wasn't expecting the resistance on the stringer and I pulled the router up. Needless to say this left a gouge about the size of the bit outside the plug outline. Any suggestions for filling in this void/hole around the plug?
Yep. routing's scary until you get used to it. Just go slow and stay strong on the router next time...
You can install the box and fill the hole with extra resin, put some of that foam dust on the floor in the hole (after installing the box). It'll take up some space and require less resin..
If you want to cover the mistake you can paint the foam a dark color in the fin area before laminating.