Because it has a few distinct advantages, I've been using the California Pop Top developed especially for the smart on mine.
Car Covers, Car Sun Shades - California PopTop
-It is a one piece affair that covers all windows, so it is easy to put on. I can cover all windows in about 30 seconds (faster if I'm in a hurry).
-The material, Tyvex, has some wild properties: it keeps the car COOL, even in the bright sunlight. I have had people reluctantly place their hands directly on the cover on top of the car on wickedly hot days. Their reaction is always one of astonishment at how cool it feels.
-It keeps the heat out in the first place. Other shades attempt to reflect back out as much heat as physics will allow. (Ever notice how hot the shades are when you gather them back up?)
-Easy to stow in the tailgate cargo compartment.
-It keeps the exterior a bit cleaner.
We are so impressed with them that we are working with John Allen to develop some big ones for our helicopters.