starting with a generic mask from the craft store and building some sort of clay up around it to get the desired shape sounds like a good way to go. Make sure to incorporate as wide of an air passage as possible. If your breath just swirls around inside the mask over and over, it'll cause you to, um, die, or something.
I'm told that Crayola Model Magic air-dry clay is good for this sort of thing. It it can be sanded once dry or you can just work is smooth with modeling tools while it is still pliable.