My guess is that they just worked on it really hard, just like someone that uses Pepakura does. They find some cardboard, cut it up, and if it works then leave it, if it doesn't then change it. <-- My theory of life btw
people look towards card board as a cheap way out "usually" and their final outcome looks pretty cheap "usually". my opinion is take a little time and effort and your armor will turn out great whether its cardboard, pep'd, or molded.