When I was station in turkey I was required to keep one in mine.
It was a long and skinny one it slid beside the seat and the sliders on drivers side fit nice and very accessible. I think i still have it. I'll snap a pic for ya if so.
I have a small one that I could slide into the rear seat through the center. The nozzle would fit right under the back rest of the seat. I guess you could mount it in the hatch, onto the panel that slides up and down and covers the rear of the back seat. It would be a pain to get to it, in case of a fire though.
I kind a like the idea of having one in the glove box.
However, the really ideal place would be in front of the passenger seat (actually, attached to it).
Anyway, I think I will keep two fire extinguishers in my car, just in case.