Thanks for their input everyone!
Since posting this request for info on SOMA I've since read a few more forum articles, one guy was Bagging the Soma Bags

but not for increased air pressure as Ali pointed out, but for pierced tubes in transit and having to repair them on the run

I'm about to make a start on Ali Cat's brilliant POOL NOODLE concept... if it works out I'll post some evidence.
Cheers
Gordon.