IMO anything in side the cabin is a waste, and actually more dangerous than having nothing.
My thought process is say, "HOLY CRAP!", you just noticed a fire in the cabin. The cabin is a very small enclosed space with a ton of plastics and things that if they burn will do massive damage to lungs in a short period of time. The only safe thing to do is to exit the vehicle ASAP, where every millisecond counts, and not be hunting for and unlatching a fire extinguisher to save a vehicle that is protected by automotive insurance anyway.
Second, I worry about leaving a pressurized device inside a cabin that if it releases its contents unexpectedly could prove a hazard (might blind you on the highway), or could be sensitive to the extreme temperatures at least in Southern states of the US where the greenhouse effect of the glass can cause temps to potentially reach 130oF (I've had black plastics on the dash melt before in Texas).
So if anything, I'd have it in a rambox if equipped or maybe somewhere in the bed, and if anything I would think the bed would be COOLER than the interior can get, since its unlikely to get above ambient since its not a perfectly sealed environment and is in the shade.