I keep a standard fire extinguisher in each of my Mustangs. I didn't want to mount them in the car so they are both screwed to a piece of wood, about three inches thick and a little longer than the extinguisher. I keep them on the floor just behind the driver's seat. Handy to grab in a hurry if needed.
You might want to hold off until the August Mustang Monthly comes out. There will be an article on fire extinguishers in it.