Putting aside, for the moment, the environmental friendliness of materials, since finding a truly environmentally friendly material seems a tough job, unless you resort to sealskins and sinews again.

A framework similar to a SOF kayak would help quite a bit with stiffness, with multiple station forms. I like to rip 1x2's in half to get 1x1's for various things I've built in the past, and these could be used as stringers between the forms. Then strip it with strips of corrugated cardboard with the corrugations running perpendicular to the strip sides. then on the inside, between the stringers and bulkheads, glue some more cardboard with the corrugations perpendicular to the corrugations on the outside layer. Then encase the whole thing inside and out in spray-on truck bed liner material (or they have this more flexible spray-on "rubber" stuff, $5/16ozs., at my local auto shop). Epoxy and glass is relatively expensive.

Anyway thats the direction my current thinking is going. Maybe make I-beam stringers out of 3mm ply (or the cardboard) instead of the 1x1's. But this wouldn't stand up to that hammer test in that Necky video.