Put a large bell jar over the car and then pump out all the air? Or maybe pump liquid silicone into the battery compartment? (The theory there is you need a fla

Put a large bell jar over the car and then pump out all the air? Or maybe pump liquid silicone into the battery compartment? (The theory there is you need a flame-retardant 'foam' at very high temp but you could submerge the battery compartment in...something like silicone, to isolate it from atmosphere; there would be heat as the batteries discharge, but no combustion.