The right one looks the best... Nice camo.
Just put a chunky circle on the guys mouth, with some filter type things out of the sides...
The pictures are quite big, so i'll just give you the links.
http://www.army.mil/cmh-pg/photos/WWII/ ... 180798.jpg
or there is a modern-ish one:
http://www.southernohiogun.com/images/GAS-MASK.jpg
or some russian one:
http://www.stationhouse.com/hazardville ... 20mask.jpg
... I'm out.