Gap
You can use the some milled fiberglass, but I would use more of Microballoon and make a fillet. You want it the consistency of thick toothpaste or icing. Use blue masking tape to hold in place and not run or sag.
If you want to email some pictures to me, I can see what you are up against. My email is the same s/n: rcflyerdan at gmail.
Dan