Hi. I managed to get a pretty solid chest plate on my first try. i just put some cheap wooden dowel pieces inside first as support. ill ads pict to this reply when it gets finished but the end result looks pretty good i guess. You can get a pack of 10 at walmart.
BTW qhen supporting the pepukura, i'd advise making sure its supported vertically AND that the vertical supports are LINKED to the horizontal supports. Make a kind of complete "Skeleton" that will hold up crucial points such as major edges, length ends, corners etc.
EDIT: As a side note, you may want to try using glue to make your pep more rigid. i just remembered that i usually use excess amounts of super glue in the valley ridges inside and outside to make the "Model" more rigid.
BTW if it's ok w/ you, i will be stealing your idea to scratch build the side equipment plates. lol.