If you want it to look like it came from factory when finished it will have to be flush face with existing glass.I assume the plywood you have behind it now is temporary just to hold it at the right height.You could make templates of the gap between existing and lay up outside the boat and then cut to template. Put in a couple of screws to hold them in place,bevel grind top and bottom.glass the joints.Remove the plywood supports when fiberglass cures and then fiberglass the back side.Should be seamless once you use your drywall skills filling the joint with thickened gelcoat and sand,fill,sand again. etc..