That Poly Performance mount looks pretty neat... a little pricy though (considering multiple vehicles). I saw a thread in a 4x4 forum once....where a guy took an adjustable "water bottle" cage/holder made for bicycles that he bought at Walmart and the smaller sized fire extinguishers fit perfectly. I believe it had a rubber strap on it, the mounts they come with are pretty flimsy.
A long time ago in a far away land.... "somebody" forgot to put the oil fill cap back on an Toyota SR22 engine I was driving. I pulled into a Checker Auto Parts to inspect what/where/why there was smoke starting to come from under the hood.... opened the hood and ...whoosh..... flames. I dropped the hood down (not allowing it to lock) ran into the store and came out within a minute with a large fire extinguisher and it went out quickly. I don't know why anybody would not try to put it out.... fight or flight I guess.
Everything was fine and I went on my way... I now carry a fire extinguisher in every vehicle. At the very least to help anyone else out. When I took my C.E.R.T class I remember the Fire Captain showed us how to work the nozzle in a back and forth (left to rightish) or circular motion and it was very effective. Much more effective at putting a large fire down than just aiming for the center. For what it's worth;)