I haven’t made any doors in fiberglass yet, but what I would do is to lay a 3 ply buildup of carbon fiber inside the wingtip in the area before you cut it open. Then cut the door out, leaving a few small “tics” where you don’t cut it yet. Then put clear packing tape inside the door that you are going to cut out, and lay 2 or 3 layers of carbon strips along the cut lines to make a flange that won’t stick to the door. When that’s cures, carefully cut the “tics” that were left and the door should pop out of the hole, leaving behind a flange. The carbon should keep the door very stiff and will have the exact curvature of the wing from where it was cut from.