Not to say that DaveW's neoprene isn't doable, but in boats there's the Lloyd's standard which dictates steel brackets for fire extinguishers.
This, of course, is so it will stay in place, regardless of knockdowns or UV light and salt water degrading plastics.
I know that a car is hardly a boat, and it may be overkill, but I kind of like the idea that the extinguishers stays in place, even in the event of rear shunt or a roll.
You can get some nice stainless (or black powder coated if you want) holders for extinguishers. It might make it look "the bizz".
As for placement, I don't know.
Okay, maybe I'm just overthinking these things here late at night.
