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I have mine on the floor clipped to a bracket that is mounted to the front 2 passenger seat mounting bolts. It works and isn't really too much in the way for passengers.
They make brackets that will mount it to the front of the seats as stated above. Personally a better place would be in front of the driver's seat. You want to be able to get to it even when you're buckled up.
Get the Brey-Krause extinguisher bracket. It comes in black krinkle finish and looks great mounted in the car. It uses existing bolts at front of either seat. No modifications to the car.
I put mine on the passenger side since I would have a navigator when road racing. I put the extinguisher bracket towards the inside so I can reach it from the drivers seat if necessary.
I think it is easier to get to it than to try and get it past the steering wheel if it was mounted on the drivers side.