Here are the BASIC rules to avoid FMCSA (DOT) Violations on Fire Extinguishers:
The fire extinguisher must be :
located so that it is readily accessible for use
securely mounted to prevent sliding, rolling, or vertical movement
Where to Mount the Fire Extinguisher?
The best place to mount the Fire Extinguisher is on the floor, just in front of the passenger seat. It's easy to get to from the driver's seat in the case of a head-on collision.
Buy a proper mount that was designed to allow you to easily remove it in case of an emergency. There's no time for fiddling with Allen keys or a socket wrench when stuff's on fire.
The Fire Extinguisher must be tightly secured. If it's moving or rolling, that is a DOT violation - and by the way, this is the most common fire extinguisher violation!