Before you mount it on the trunk bulkhead, how thick is the bulkhead sheet? If it is only 16g (1.6mm / 1/16") sheet alum. alloy, you would need to use a correct thickness "washer plate" on the trunk side to prevent the heads of the mounting screws pulling through the thin bulkhead. In a mild 10g deceleration, all of a sudden your 1kg / 2.2lb extinguisher becomes in effect 10 times that weight and the screw head would pull through a thin bulkhead.
Good idea to carry the fire extinguisher, but it needs to be mounted so that it's safe in all circumstances.
Cheers,
Glen