I have used some of this syuff before, not for what you are using it for though but simmilar. I don't know a lot about its specs but it is hard and light weight when it expands and cures, but its messy so be extra careful. it will stain whatever it gets on forever. i would do some test first before you use it in your armor, like make some small boxes with your foam board and put some in them too see just how much it expands. and it does get fairly large so dont fill the box just a little. i hope this was helpful. I am learning alot here, but i know a lot about trying to figure things out on my own with new materials. i am experimenting all the time with new materials for all my art.
Eternal