I've been thinking a lot about how to make the bellows.

Then the other day I noticed this detail on one of the screenshots from Kill the Moon I'm working from.

Can't see it? Let's take a closer look at Capaldis Helmet.

Is that... a seam? Or a patch of a rip perhaps?

Anyway, if the originals have a patch or seam like that then I could use something that I join at the back. Like an accordion boot from this supplier: 

https://www.accordionboot.com

So I ordered some to try out. These things aren't cheap but at least I could buy per feet and not a ton of it I'll never use. ;)

Now, let's get back to the area at the back of the helmet again.

When they moved the air coupling at the back of the Procap helmet originally they left the slot for it open like this. Perhaps it lets air out as adding a rubber bellow hinders the normal overpressure way out through the stretchy liner under the jaw. It has to get out somewhere or air won't really circulate in the helmet.