The reason for the “metal-to-metal” requirement is that the fire extinguisher is a heavy object and we have had instances of the plastic brackets failing. The Extinguisher then becomes a projectile and dangerous to anybody in the vehicle. I agree that the factory mount should be strong enough, but it is a risk that we, and the insurance company that provides us with our event insurance, cannot take. We have had an instance where an extinguisher broke loose and hit an instructor in the head. If it were not for his Safety Helmet, he would certainly have been injured.