Saw this online. While I'm not into the final design per se, the process that he got there is quite interesting. Just another example of ingenuity and resourcefulness applied to building a car.
It appears to be a 2 part (exothermic) expanding foam, similar to that of the expanding household insulation you can get in a a spray can. I used to use 2 part urethane "fast cast" compounds like this in college for building models.
http://justsomething.co/old-junk-car/