So I decided made my own for less than $5 using:
1 A4 silver card
1 A4 black card
1 A4 grey card
A length of string
And masking tape (no pun intended)
Step 1: The Pattern
Sorry the background is red, couldn't find any black card to place behind the shapes. But you can just adjust that in your computer so it doesn't murder your printer.
Cut the shapes out and trace them onto your card, but not on the side that will be facing outwards. Don't want messy pencil lines on your mask.
Now feel free to modify the pattern as you like, as it was made to fit my head and Doom's mask has many variations.
And I suggest you do a test version on normal paper before you go and make your final one.
Step 2: Almost Done
The 'cheekbone' is attached behind the 'cheek' and the 'face'
The 'grill' is glued onto the 'mouth'
'Mouth' is then attached underneath 'face'
The 'chin' is folded to create more dimension and attached behind the 'face'
Glue on the silver rings around the eyes
The nose is the trickiest part, you have to push it in from behind the mask and you might need someone to hold it while you tape it down.
Now punch some little holes on either side of the mask in line with the eyes and tie the string through.
Below is the inside of the mask so you can see how everything is taped together.