I actually got a large roll of BLACK foam, and I didn't realize how lucky I was at the time. I used it for all the "trim" around the edges of the armor pieces (cut 2" wide strips of it, fold it in half while ironing so it's a 1" wide strip, hot glue that closed, then cut triangles into the open edge so it will go around curves, and hot glue that to the underside of the armor edges. Not screen accurate, but looks great). I haven't found it (black) anywhere ever again, so if I run out tonight on my last piece I may need to figure out how to duplicate the correct color with the white foam instead.
The surface decoration is affixed to the aluminum mostly with Elmer's glue stick. I know, shameful, but it gave me plenty of time to work (unlike hot glue) and sticks pretty darn good. A few places that have pulled up due to the curve I put on the piece have been repaired with super glue, but that often leads to crazing that I then need to cover with grime. I bet some kind of contact cement would have worked, but I went with what I had on-hand.