[Bicycle Bob]

I did the math for a foam-core monocoque velomobile, and for the required strength, the skins were about as thick as good paint. What they really need is toughness - maybe aramid fiber over airex foam, to minimize the extra weight needed to resist tearing and punctures in the real world. You have to think more about how it will flex, and try to take advantage of both the rigid and flexing aspects of the overall shape.
In a crash, carbon does not absorb much energy before breaking into bits, whereas a Spectra or Aramid structure will take a beating slowing you down and turn into a protective bag. They may need a few runs of carbon or 'glass in the compression zones.