"I keep mine loose on the front passenger floor...."

If you have carpeting on the floor of your vehicle, using strips of the industrial strength velcro fasteners (male strip I suppose) can help keep the unit from rolling around. They can help to keep the unit in one place and it is not that hard to pull it loose when needed.
I use the self-adhesive strips on one of the extinguishers I keep in the trunk of my car. Not a solid tie down by itself, but positioned properly, it keeps objects from rolling around.

In Brazil, where traffic can be highly congestive, preventing the prompt arrival of firefighting equipment, it is mandatory that a fire extinguisher be within reach of the driver of a private vehicle.

FWIW