Now, here's a Very Light Vehicle:
Driving the Spira 3-wheel prototype
You get enough strength to just get down the road from a box made of common styrofoam insulation with fiberglass tape reinforcing. The Unobtanium I'd like to see developed is foam with dispersed fibers to raise the physical properties. You can start your structural design as a whole-body bike helmet, and then see what else it needs. If it is plenty tough but not rigid enough for modern chassis tuning, a subframe can maintain wheel alignment, as on the Model "T".