Stephen,
Hey, this is bringing back lots of memories! I did a similar diorama about 4 years ago in 120mm. I'm at work right now, but I'll post pictures on my vBench when I get home (say in about 4 hours from now). I too am curious about the scale. Also, is there any chance you can save a casting for me? I'd go to the show just to get my hands on a casting, but Italy is a bit out of my way
A couple of suggestions, if I may:
1. make the beams on the hedgehog longer, particularly on the ends that are sticking up. They were typically made out of railroad junction rails (the ones that slide side-by-side to switch trains from one track or another) or by actualy rails themselves (looking sort of like "I" beams). These were pretty long, with the bottom ends sinking deeply into the sand. However, the exposed ends would be longer. When I did mine, I examined pictures of the "real things" and worked out the scales. The beams came out to close to 8 feet in length.
2. the M1 helmet should be a bit taller, but just a bit.
A good reference for such a project would be the "Saving Private Ryan" photo-book. If you can get your hands on one, do so - it's filled with very good pictures of the opening scenes. If not, let me know and I can scan some pictures for you.