The key feature for the jaw are the lips and metal re-enforcements. I think that once you get these in place you can have whatever details you want on the rest of the mask.
The lips are yet more cardboard, cut into shapes and glued onto the front of the jaw piece. I tried to mimic the photos I had of the movie prop but in the end I just added these things where they looked best. Each vertical bar is square and cut to line up with the top of the jaw. The re-enforcements on the bottom jaw are all tapered down to a point, the lower Jaw was also cut into a wave shape to add some more features.
This basic framework for the jaw trap was then covered with PVA glue and newspaper (apologies for the lack of these photos but If you're that desperate you can check out my other instructables for photos of how it might look) I dyed my glue a dark grey colour using poster paint. This made it easier to get a good solid colour across the mask.
To give it the metallic effect I used a metallic poster paint and a dry brushing technique. There are many better guides on how to dry brush, here is mine should you decide you dont want to look elsewhere for on.
Dip the brush in the metallic paint. Wipe the brush on a tissue or peice of cloth until most of the paint is gone. Then brush the object lightly with the brush. The metallic paint sticks on the raised parts of the object making them look like worn patches on a metal object.
This is the basic mask, from there you can add details.