Reference pictures from the "Star Wars: Behind the Magic" Multimedia CD show that the snowtrooper was custom sculpted completely out of clay at one point. The prop/costuming community has not found a suitable shaped helmet that ILM might have based the Snowtrooper helmet off of, so we assume the snowtrooper helmet was a custom piece.
The Snowtrooper helmet consists of:
Details, as seen in the Reference Section of this site:
FYI... General Veers field helmet was actually a snowtrooper helmet with a comm unit attached to the side and rubber trimming around the edges. The same helmet style was used for the ATST drivers in "Return of the Jedi".
The faceplate, or goggles (as some people call it), Has a distictive looking bill to it that allows the vinyl cowl to protrude slighlty forward of the helmet. This protruding creates the illusion of a breather mask under the cowl, presumably used for cold weather breathing in those frozen areas like the planet Hoth.
When sculpting the faceplate, remember that there is a distinctive ridge just below the eye slits. Alot of propmakers missed this ridge in the early days of Star Wars costuming, but since we all have closeup reference photos in the Reference Section, you can see how the ILM faceplate is correctly sculpted.
The Vinyl Cowl
Here's a great helmet assembled by a member of the 501st Costume Club, Jay Harris (TK-1010). Jay's cowl is made an ultra slick sheet of vinyl that he picked up at his local crafts store, "Ragshop". Ragshop has a website, but they do not have an online catalog to order from. We are attempting to find an online source for this excellent material.
For now you will have to make do with using regular textured vinyl that you can find at almost any cloth store. Perhaps you will luck out and your local cloth store will carry the ultra slick vinyl, but the costuming community has found that to be a very rare case. The ultra slick vinyl does not sell well to the general public so it is tough to find.
Go to the Armor Assembly section of this site to see how the Snowtrooper helmet is assembled.