Another approach by board folk is to make a hollow extruded polystyrene board, using 2" xps (sic). That way you can make a torsion box (or boxes) that are mainly air ( like the IC with the blue foam above ). Xps is good in compression, but little else. More surfboards are using xps inside for the spine and lateral ladder type support. Then epoxy & cloth around wood, which is itself built around the foam. Sometimes balsa. Same sort of thing for some A class cats, although glass reinforced wood for the main bulkheads and skins. Considered kind of old school, methinks, so same problem as new IC's, weight and stiffness wise.
The hollow board with both inner support and skin of xps solution does leave you the delights of shaping the stuff, although once you get it Shaped right, you have to scuff it up with 200 grit or so so laminates will stay stuck.
Laminate a piece of thin ply on the dance floor. But there is still the guessing game on how much stuff is enough to deal with the torque between seat and rig. Swaylocks forums have many posts concerning boards on all these methods.
I support the 50kg rule, and I can't use epoxy! Perhaps throw away hulls are the answer, if they are cheap and quick to put together. Recycle the hardware from hull to hull.. It would give regattas some added spice. But you would have flotation if you used xps.....