The plan is to use the piece I've built as a vacuum forming buck. If you don't know what vacforming is, it's the way stormtrooper armor is usually made. It's also the way Sean Bradley makes ODST armor and the way I built my HALO3 Marine kits. You heat up a sheet of plastic in a frame until it's stretchy and flexible, press the frame over a form, then suck the air out from under the plastic. When the plastic cools, it stays in the shape of whatever was under it.
I've got some pictures of that process somewhere around page ten in my thread about building my H3 marines:
http://www.405th.com/showthread.php/31388-Thorssoli-s-HALO-3-Marine(s)
That should make it all very clear. In the case of the health pack, I'll be making them in either one or four pieces. One piece if it's just going to be set dressing to hang on the wall. Four pieces if I make one that can be opened with an inside and an outside. I'm also going to have a friend of mine run me out some vinyl cut decals for the labels on the outside.