His thought is that it was made of "gaffer's foil" (the foil used on film sets to shape light or make improptu flags) wrapped blue lighting gel material.
I have some of that material here at my studio, I can't think of anything else the dooms day machine could be made of.
Seems highly plausible.
May I suggest an experiment? If you have that material on hand, would you be willing to try to create a replica of the Planet Killer and post photos for comparison? Maybe the way to answer the question of how it was built is to try to build one that looks just like it.