Just don't leave the extinguisher loose. Keep it strapped into it's mount which is, in turn, bolted to the vehicle.
Actually, I think the original post exhibits a pretty good basic kit for a car. I carry substantily more, but I'm driving a full size 4-wheel drive pickup with a cap and I'm in a differant area from the author.

"There is nothing so frightening as ignorance in action."
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"There is nothing so frightening as ignorance in action."