As well as using a tight weave glass and Paulownia, I've used thick minicell foam as bulkheads. I figure that not only is this really light weight (and commonly done on commercial plastic boats) but it also allows some hull deformation if you hit a rock. The Black Pearl I have is only 12kgs using this approach.

In use I've noticed that the tight weave glass doesn't wear as well as normal, heavier thread glass - it's not so strong so needs doubling (or tripling) in tough areas; and paulownia dents easily - again, not as strong at resisting impacts as wrc.

In a few years time I'll refinish the boat and I might split the deck and hull apart - I've wondered if the foam might trap water against the glass and over an extended period of time would this damage the wood? Splitting the deck off would give me a chance to see.