Actually, you've just reminded me of that. I never really considered making the mask rubber; I did years ago but managed to forget that it is a gasmask and should be rubber. :rolleyes I did find a video of these guys who have a Civil Protection mask made out of rubber.
Link:
http://youtu.be/0Tp4k5MlG9g?t=57s
I did later realize that the mask isn't really rubber; (cannon) or at least, I am pretty sure it isn't based off of this:
http://youtu.be/91uWeqArrok?t=2m49s
I don't know about the Combine helmet though, but I assume that it is a bit more than just rubber based on the way it would need to stop bullets. However, the concept art did show a rubber gas mask for the original Combine soldier.