Completed fitting to the car.
Got the Bunnings $22 extinguisher (has a tight grab on the neck of the extinguisher - no traditional wrap-around locking clamp) and some nuts, screws and washers. Drilling the holes and fitting the extinguisher holder was less than 10 minutes. Bolted to the car in 5 minutes.
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Looks great but unfortunately it does interfere with the seat adjuster. Simply put, I need to have the seat all the way back and can barely move it forward two notches before it won't lock anymore. The above pics show the seat full back. Here is a pic of it as far forward as I can get it:
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Overall still happy, but I think the extinguisher would better fit on an angle rather than flat, and maybe extend slightly further down, but then this would likely interfere with the hump under the carpet. Of course if my passenger seat was electric or didn't have a horizontal adjustment bar, it wouldn't be a problem.
I was going to procrastinate, but I put it off...