Everyone seen those reflective things you can buy for the windshields, they look like bubble wrap between foil? You can buy rolls of that stuff at lowes/home depot. What you can do it buy a roll of that and line your whole trunk with it, espically the top/lid. I fif it in a truck, you cut panels to fit between all the structural steel then use aluminum duct tape to hold it in place, works very well to reflect the heat outward in the summer or inward in the winter. I did the whole cab of my truck and would not turn the heater on sometimes in winter as it would reflect enough of my body heat to keep me warm. I was going to cut panels out to cover the windows and tape the edges for use if I ever needed to shelter in the truck.