Innertube bands

[Bendy]

It's not the first time I've asked a bizarre question on here, but really could use some advice.
I have 4 British m76 lightweight para helmets ( one of which is a nice early smooth version with the wight/cream leather ) and I would really like to finish them off with an inner tube band.
The problem being I have absolutely no idea of the size inner tube needed for the 'contract'.
I've had a swift look on line that has left me hopelessly baffled!
Any assistance would be greatly appreciated from the more technically minded folks out there!

[combathelmets247]

NBC boots were used, snip the top half of the boot and you have your band

[James C]

Bendy fire us over a PM if you get chance as i have a couple of spare cut bands you can have with pleasure