I like Ron's double-skin "surfboard" layup with a light
core such as foam or balsa. Torsion boxes are the stiffest thing in the world for weight
. I also agree that framing for the mold is important. Any sort of lamination that you can do quick and dirty over the opening and then remove for layup should be supported to avoid going into winding (twisting). This means a frame or cradle. Doesn't have to look good but has to provide a stable support that doesn't lose the shape due to gravity sag and torsion.