When I was an aviation photographer I spent most of my time on various flight lines both military and civilian around the country in my work truck. I had a 20# CO2, 2 5# Halons and a 10# ABC mounted in the bed and easily reached if needed. I also had a 5# Halon mounted on the floor in front of my seat and a 5# ABC on the other side. I realize this seems like over kill Sometimes I was the only one within 5 minutes of responding, if something expensive is on fire there is no such thing as too much.
I have 3 5# extinguishers in my Willys. I have a 5# Halon (1211) and 2 5# ABC (3A, 40BC) mounted in steel brackets with rubber straps. I used to have a 5# Purple K but kept getting it and the 5# ABC confused so I went with 2 ABC's.
Yup, I know sounds like a lot of overkill. After the fire is out I have never once heard anyone complain about too many extinguishers or having one that didn't get used. Once things get to burning too much is just about right, especially if it's my stuff.
Here's a pic of the brackets I use, easy to use and secure.
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