Adhesives need surface area to work properly. Take a look at "composite nuts" or "plate bonding nuts". These are designed to be laminated/sandwiched inside a composite material and are essentially nuts with a plate welded to them. You can replicate it with a wide washer, a nut, and a welder. What I would do is buy/make one of these, bond with adhesive, and hand laminate 3-4 layers of fiberglass/epoxy on top of the flange, increasing the area of every layer.