Step 1: Materials
Craft foam(once again, Hobby Lobby- 79 cents)
A good plan, and probably use a drawing.
The picture is the foam, which is 2mm thick, I believe.
Step 2: The idea
If you have a turned(3/4) view, you will want to draw the front and back as well.
Unnecessary note: I later scrapped the "gas mask" look for the "made in one night" look.