Unless you require the driver's seat to be all the way back, you can use the
hinged cover on the compartment under the driver's seat.  I have mine mounted
there and it's readily accessible from the driver's door - it's mounted in
the horizontal and it's one of the big ones (maybe 4-5" in diameter with a
flex rubber hose).  Several years ago I assisted in putting out a car fire
that took the life of a 22 year old girl.  Numerous motorists had stopped and
there were perhaps 5 smaller extinguishers being used.  We'd beat the flames
down and they'd come roaring back.  Never could get the flames completely out
until the fire department arrived with heavier equipment.  I've vowed to
carry two items with me ever since.  First, at least one good size fire
extinguisher (I also have a halon extinguisher, but I don't believe they're
available anymore, and second, a non-flamable blanket.  I hope I never have
to use either again (except the blanket for the occasional picnic!).

Ken Hartz
89 White Wolfie
87 Blue GL (gone but not forgotten)
85 Tan GL (my intro to the best vehicles on the road)
84 Audi 4000S
80 VW Quantum (  
61 VW Bug
55 VW Bug (Huge gas cap ... small engine)