I have mine bolted to the floor behind the driver's seat. You should be careful with anything to put in front under the front cushion so that it can't get free too easily and end up under a brake pedal. I just couldn't figure out a way that was secure enough to do that (like bouncing around off highway and I occasionally would kick the lever getting in and out of the truck) without being too hard to access quickly to actually put out a fire, which is why it ended up behind the seat.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
“Anyone who believes in indefinite growth in anything physical, on a physically finite planet, is either mad or an economist.” -- Kenneth E. Boulding