You can glass the wooden framework to the hull with fiberglass tape and resin. You might even get by with a strong adhesive like 3M 5200 to bond the wood to the hull. If I were doing it, I think I would construct 2 or 3 upside down trapezoid shaped frames to fit the hull shape and bond them in place so that the frame itself bears the weight rather than your hull to wood bond. Top off with epoxy sealed marine plywood in a V berth shape and maybe cut out an access hatch so you have storage underneath.
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