I really reccomend an onboard fire extinguishing system. A puny 1 litre fire extinguisher mounted to your roll bar isn't going to help much in a big fire. With this system you simply pull a switch which releases SCCA approved Halon in key areas where a fire might be. The halon can be cleaned simply with water.
You can get a full on halon setup just like the big boys for less $400. I thought the same as you get a small extinguisher. But after a crash depending on how you land. You will be knocked around and then looking for an extinguisher. Its much easier looking for the big button that says FIRE. Plus you are going to need a mount that keeps the projectile from flying through the cockpit.
I just mounted a fire bottle in my '94 Cobra, it's a hand held Halon unit from H3R out of San Rafael, CA (Bay Area). I think I paid around $150ish delivered from www.h3r.com.
To attach the fire bottle securely inside the car, I made a bracket from 2 pieces of stamped steel readily available at the local hardware store. The homemade bracket bolts to the fire bottle's quick release bracket. The fire bottle and bracket assembly then attach to the passenger side front seat mounting bolts:
I could probably shorten the homemade bracket a bit and bring the bottle in close to the seat, but the most important part was getting it mounted.