He puts the Reflectix only on the windows? It makes sense that the windows would be the place where you'd lose the most heat but I never would of thought of selectively insulating the vehicle. Clever. That plus the rope between windows to cut the space in half would really create a space that can be heated with whatever source you have for twice as long.

And if you have Reflectix or a reflective windshield screen you can lay on top of it to prevent heat loss where your body contacts the car. Even though the seats are insulated, the Reflectix would bounce the heat back to you. Add a space blanket and your body heat alone would last longer.

Add a heat souce such as candle or the propane catalytic heater, or even periodically running your engine (assuming you clear the tail pipe and have enough gas and battery. Be careful to ration your gas and battery. They can be used with foam from a car seat to make a black smoke fire to attract SAR searchers.