I'm actually wanting to do a Dark Helmet cosplay as well sometime. For the movie, it was probably constructed using vacuum plastic forming, which is out of reach of most cosplayers.
I haven't thought of it all the way through, but I was thinking of using chicken wire or mesh for the basic shape. Then duct taping the whole thing. Doing a layer of Paper Meche and fiberglassing over that. Then using bondo until it is perfectly smooth. As for how to make the front flip up, I'm not sure. Like I said, still haven't thought it all out just yet.
A Dark Helmet cosplay would get attention / glomps at cons
Pen Pen (Evangelion) - 10%
Duke Pantarei (Tales of Vesperia) - 95% - wig needs restyling.
Cecil Harvey (FF Dissidia) - 10%
Emperor Gestahl (Final Fantasy VI)
Rakushun Rat form (Twelve Kingdoms)
Shulk (Xenoblade Chronicles)
Link Zora Armor (LoZ: Twilight Princess)
Desired, but not confirmed:
Dart Dragoon Form (LoD)
Dark Helmet (Spaceballs)
PM me for free photoshoot in Seattle area.
My Flickr page: http://www.flickr.com/photos/finalevacosplay/