I've got a crazy idea....Well first would just be a Balloon....That would be sweat and not comfortable at all, but it would give the strechyness that you are looking for.....but it would prolly suck
So the actual Idea that is less crazy...well less crazy after the Balloon idea......is plastic. for an example I will fall back on my Clasic....the Phantom.
Okay that makes little sense with that example...but find someone that can mould plastic....well you can do it yourself too easily enough. But get a piece of plascti that can come down from the Headpiece and have two ear holes...make sure that they don't have sharp edges otherwise Wiccan will get cut ears...same for the outside and his cheeck...or the ear holes couls be larger than needed to ensure room. And have it come down the sides and back of his head. Then have a second piece to come around his neck...attache it with soem 2 sided tape or get a set of snap buttons on it.....or the neck part could still be material.
I actually think form fitted plastick might be the way to go.....but hard to do.....or form fitting rubber....there was someone here on the hype before that made cool masks......I'll see if I can dig up who it was...he was gonna make me a Roughneck mask and it wasn't too expensive...he should have some ideas on this at least.