Another thing you can do is add some epoxy on the underside where the slot is. Not a lot, but maybe add a very little cabosil or microballoons and create a small fillet at the fiberglass and wood intersections. The filler would only be used so that the epoxy stays where you want it. Cabosil is more thixotropic and would be my choice.

I agree with Bill that you need a little more resin because the cloth provides the strength, and the amount of resin controls the stiffness. Try adding some resin on top first, and if that doesn't do it, try my suggestion.