I made a bracket that attaches to a commercial Amerex Bracket 860
It mounts to the stock seats. It bolts to the underside of the seats where the seat bolts to the seat rails. No drilling or modifications are required to the car or seats.
The bracket I made is tig welded and Painted with Black hammerite.
Here is a description of the Amerex bracket.
http://fireextinguisherdepot.com/brackets/amerex-bracket-860.html
Heavy Duty Rubber Strap Bracket for Extinguishers with 2.75" -4.5" Cylinder Diameter
I replaced my stock seats with Recaro seats. I needed to make a different bracket.
You will need to supply your own fire extinguisher. I was using a 6Lb Halon extinguisher.
If interested I can take pictures.
Later,
Don