Most extinguisher mounting kits are only fit for the static weight loading and forces of the unit itself. In an accident in a vehicle the extinguisher becomes a lethal projectile.
Where ever one places it one must consider what happens if it breaks/comes loose. This rules out drivers side, rear parcel shelf, upper "B" and "C" posts for starters.
Front passenger footwell (lower "B" post below dash or centre above feet and below dash) are ideal. Next best IMHO is below/under front passenger seat. If it comes loose then it is a passenger annoyance. If it is released in an accident then it could break an ankle (or two) but that is about all.
Personally I now prefer the enclosed but accessible boot area. Extinguishers only last so long (5 years) and re-drilling for different mounts every 5 years does not appeal to me any more.