1 bottle, 2lb capacity, metal head within reach of the driver as they are strapped in to the seat. Zip-tied won't cut it, while behind the driver may be "within reach" you should have it in front of you or to the side so you see it easily and it's on your mind frequently. Should have a quick-release on it like what most come with out of the box.
Also, you should take the bottle out before every race and lightly tap on the bottom of up (upside down) to re-animate the content. After sitting in the same position for long periods of time the retardant will separate from the propellant and without disturbing it frequently, they will separate and when you need it you may get propellant but no retardant.