Andrew, Reincarnation had many foam bulkheads. The foam was low grade and the deck stringers were set directly into it with insufficient'engineering' long-term. The tops of these bulkheads were removed and replaced with 4mm ply I-beams. No stringers were necessary as the I-beams supports the 4mm deck, not the foam. dkd suggestion of applying (6oz?) glass to the ply will work, espescially on the underside were it will be under tension when loaded, crew sitting on it. Applying it to a flat sheet then bending it to the required curvature will place all the 3 layers of veneers under tension that may result in some splitting. Applying another layer to the upper surface will bind this top veneer. You will have to consult dkd about any support of the deck, ie. bulkheads/stringers. I can email info on the construction of the I-beams if required, yes can be slow but NO deck failures of the 4mm ply after 17 years of some serious sailing.