Ok, so I'm new to the thread, but I'm going to throw in my 2 cents now. I'm not much of a prop replicator, and I definitely don't hold a candle to any of you guys. But I AM a WWII reenactor and a Vietnam War reenactor, so I am kind of familiar with some of the "real world" military stuff. So I'm going to comment on some notes I've taken from the 21 pages in this thread. The concept helmet image posted on page 20 does look like a cross between the helmet that we now know from the film, and the Endor helmet. The only thing I think is different is the base helmet. Instead of the Vietnam era M1 as a base, I'm seeing a modified WW1 German Pickelhaube.
As for the color of the blast shield on the screen used helmet, I've seen you guys saying "green", "grey", "blue", etc... What I'm seeing (and this may just be me, but I think its close) is a weathered WWII German Panzer Gray.
And finally, not saying these are the same bands used (from the screen shots they look like they are cut perfectly by a machine, like the previously mentioned sports bands), but here is a suitable replacement. As pictured, these are exactly the size needed for the M1 helmet. From screen shots, it looks like most helmets have 2 bands to me.
These can be purchased for $4 a pop here:
http://www.mooremilitaria.com/helmets-body-armor/band-helmet-inner-tube-usmc.html
Now, I gotta tell you... You guys absolutely amaze me. Seeing all of you come together in such a short amount of time, sharing knowledge, utilizing your individual skills, and coming up with such an amazing replica, all from some pretty low res and crappy reference pics just absolutely blows my mind. You guys are awesome!