Ive been considering the same problem, I want the extinguisher to be readily accessable from the drivers or passengers seat, but not get in the way of things.
So far the "spots" if considered are the passenger side kick panel, the back of the passenger seat (either mounted directly to it , or with an under seat bracket) , and lastly on the rear seat kick panel on the drivers side...
As for using underseat brackets, ill have to look and see if that will interfere with the operation of the seats. i may just end up getting another small extinguisher and mount one on each side somehow. so those are my ideas so far...

side note: you never know what can happen.... i hope everyone has at least one extinguisher in all their vehicles , the only thing worse than being on fire is watching someone else on fire..either way, a $15.00 investment can save lives, and allot of pain....