I bought the cheapest sunshade at Wally World and cut it to fit. It is very thin foam with a reflective foil on one side. I used high heat aluminum tape to keep the cut edge from fraying. I keep it folded up in the trunk during the winter, then place it between the sunshade and the glass in the summer. It works like the opaque sunshades in our Subarus and I don't have to worry about it melting or spontaneously igniting from focused heat like cardboard might (very unlikely). https://www.walmart.com/ip/Auto-Drive-Lightweight-Silver-Accordion-Sunshade-1-Pack-63-x-28-5/941281506?classType=VARIANT&athbdg=L1200&from=/search
