What is the substrate and the application ?
If substrate is fairly substantial the plate could be drilled and countersunk to take woodscrews, edges of plate need a good chamfer possibly 30 deg so that you can get a decent layup when bonding the cover layers of glass(will prevent air pockets you might get with 90 deg interface). bond the plate to existing fiberglass using your epoxy and screw it to substrate, then apply your epoxy and glass to cover the assembly.