I have also fitted one across the seat runner mounting bolts by redrilling the provided mounting bracket,but mine is on the driver's side.
There are 3 advantages to this position, it can be grabbed as you exit the vehicle, it creates a useful barrier where you can put things behind it without any danger of them sliding under your feet and thirdly because it doesn't encroach on SWMBO's space. There is one downside that it gets in the way of your knuckles when moving the seat forward but this was solved by putting a strap on the seat adjuster bar.This means that when SWMBO is driving she can still get the seat forward enough.
I have always had an extinguisher In the car, partially for competition reasons in the past, but in 50 years I have never used one in anger and only drawn it out once.
