I use rolls of the "stick on" type magnets to attach to the metal parts of the vehicle so no permanent placement- I just leave the cover on the adhesive side.

I used that in My Element for camping with a mosquito net over the moonroof and back hatch, held on by the magnets all around. When I gave away my Element, I sent that along with it, so no feedback on long term durability.

For the space blanket scenario, wouldn't it reflect the heat away? Maybe place it at an angle to reflect the sunlight in and concentrate it?