The pack is made from the same housing insulation sheets (polystyrene) I used to make a couple of packs a few years ago. I'm building it in layers, then sanding the edges to get a consistent surface.
I expect the pack is going to take a pretty good beating, so I'm not going for ubber accuracy (the other kids won't know the difference) and if it gets demolished it won't represent gobs of my time. So far the pack's taken about 3 hours.
Here's a pic of the 3/4 scale pack next to the buck I've made for vac-formed packs:
More pics to come...