Morris Minor Owners Club

Has anyone fitted a fire extinguisher under the bonnet? The type that can be operated remotely?

Yes I have!
The bottle is inside the car with a spray jet positioned near the carbs. There is a second spray jet for the drivers compartment. The operation of the fire extinguisher can be made from outside the car or by the driver inside the car. The fire suppression medium is foam.
The bottle needs to be kept away from the source of the fire so fitting the bottle under the bonnet may not be a good idea especially if the metal bottle exploded.

What about the automatically triggered ones, are we skeptical that they'll never go off when there isn't a fire?

Once had powder extinguisher come out of bracket, hit floor and go off inside van cab.Very scary while driving and was still coming out of heater vents for a long time afterwards

I have two extinguishers fitted in engine compartments on the boat, both with glass bulbs to fracture in the event of fire. In ten years they have not gone off by accident; and thankfully never needed for real. In other situations, I have used a system called Firetrace where a cylinder reservoir supplies a hose which bursts in case of fire and expels extinguishing agent directly to the fire. Again, never had a false alarm but when needed, it worked perfectly.