Ooh, that stuff sounds really fantastic! If you remember the name of the foam/top layer agent, I'd love to do more research on it~
Sounds like you've got a pretty good background with building things. o: More than I have, anyway. usually I stick with sewing and modifying pre-existing objects. The only really impressive prop I've ever made was a bass guitar, which I made plenty of mistakes on.
Since you mentioned using cardboard, cardboard covered in bondo is a method a lot of cosplayers make armor, actually. o: Probably the most popular way to make armor on a budget involves craft foam/EVA foam. You can use heat to make it soft and maleable, and it more or less keeps its shape when it cools down.