I had the same issue with my fin box. Except some idiot stepped on my board in the shallows and broke my fin and pushed up the glass on the box. Easiest way to fix that is if you have a dremel tool carve out the cracked fiber glass and make a clean void but don't dig into the box. Just get a ding repair mix up some resin and fiber so you can fill in the void. Than just just use a small strip of fiber to patch over the filler, sand and finish. Dremels are great b/c you can carve whatever you want out easily. If not use a utility knife.