Years ago, I repaired a Hobie 14 deck with a similar problem and had great success.
I reinforced that area with an old sail batten turned on it's side, shaped to match the contour of the underside of the deck there and fiberglass it in place. The deck began super strong there. I added two "ribs" about 1 1/2" from the new access port (both forward and behind) spanning the width of the decks' underside to the hull.
This work is easy to do with the boat upside down sitting on sawhorses.
Use curved base access ports and dry fit them first without silicon caulking.
Than remove the ports to give your hand more room and keep the ports clean of new fiberglass.
Let gravity be your friend.
This batten became like a "rib" in an model airplane wing. Rock hard but still light weight.