I've found, in my van anyhow, that right below the passengers seat on the
forward side of the seat pedestal/wheel well is the best place for my
extinguisher.  When the passenger is sitting, it is right behind their
calves, mounted cross-wise.  Doesn't take up any room that you need
otherwise (I seem to use the space behind the driver seat, which is the
'traditional' extinguisher location.)  In the event of a fire, I can lean
across and grab the fire bottle as I 'bail out'...If something happen when I
am walking around the vehicle or if I'm stopped beside a busy road, I don't
have to get into the traffic flow and open the driver's door or cross the
rear area to reach the extinguisher...
  You should make sure your mounting bracket firmly holds the fire bottle.
In case of a roll-over accident, those can become deadly missiles inside a
tumbling van..You should also be able to quickly release the unit for
use...duh!
  Don Hanson