Don't know about car use, but your local fire safety company should have NFPA compliant wall brackets available. They're basically just a wall hook.
Yeah, I've just got hooks in my garage.
NOTE, place the extinguisher somewhere that won't actually catch fire, or near the door or something. You don't want to be unable to reach the extinguisher because the fire blocks getting to it, and you don't want to trap yourself behind the fire relative to the door because of trying to get the extinguisher. It's better to let the fire burn the structure down without trying to put it out than it is to die in the process of trying to fight it.
I just bought 2.5 lb extinguishers for the cars that came with metal straps. IHRA requires a bottle in sportsman door cars - a rule I personally disagree with.

