Something like this:

http://www.autoexpress.co.uk/picture_library/dir_22/car_portal_pic_11451.jpg

It is secured on the passangers-side with a clamp bolted to the seat that holds the exthinguisher and it's also held in place with a quick release-strap. Just to be safe.
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''It's time for Plan B...'' ''We have a Plan B?'' ''No, but it's time for one.'' -Stargate SG-1