We run "flags" such as these on a Cooper. If heat gain is an issue our plan is to adhere the design to the outside surface and that should be very effective. Yes, it does prevent you from seeing the stars, but it does offer infinite personalization possibilities. The "S on a checkerboard" has been problem free for two years thus far. That panel is glass. I guess that there could be adhesion issues to a poly panel, but the 3M material seems quite good.
Cooper Flags
Ciao