I tend to think that a good fire could make even more of a mess. I would also prefer a CO2 extinguisher, but there are quite valid points about where and how they can be used, there are also cost considerations.
I have a dry powder extinguisher in mine. The problem with the LR recommended position, on the front passenger's side kick panel, is that it only applies to the Official LR accessory.
I was able to place it high up just inside the rear tailgate door, on the panel between the tailgate and the rear side window, where it can be seen from outside too.
No need for "In case of fire, break glass" jokes, I've heard most of them already.
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Regarding fixings, I used 2x5mm domed or "pan" headed bolts with a pair of "penny washers" on the inside of the plastic trim to spread the load.