I would say a good 2 part epoxy. Read the lable some are better for fiberglass than others. I used the gorilla 2 part epoxy the other day to glue plastic to brass.
I would think you could mix them and then smear them around over the crack and then use a wet rag to take off the extra before it set up.
2010 JK added 17" rims and 32 inch At tires, hitch, Ace rock sliders, Barricade bumper,
light guards, grab handles, floor mats, Grabars, Rock Ridge hood latches and fuel door, and 2" Rancho sport lift with 9000XL adjustable shocks.