Originally Posted by Conor
Attach it with a chain to one of the anchor points in the boot.
Only place to fix a chain without some kind of harness to a bottle is around the neck, not a good idea in an accident, unless you fancy a bomb in the car instead just a large heavy object.
It would not be much easier with a ratchet strap so it's not an easy problem to completely solve.
Does anyone make a diving bottle harness that goes around the body and the neck?