Speaking of physics, the best will be a bubble wrap-with-mirror-mylar for efficacy. The mylar reflects a great deal of the solar radiation and the bubble wrap keeps the heat that does get to the mirror face from getting through to the interior.
Notably, they are also among the least expensive. You can get one that approximates the shape of the car and scissor-trim it to your desired exact fit and maximum coverage.
If you wish, you can also buy one each for the sides and rear and still be out less than a hundred bux.
Mine is so effective (just windshield, and a loose cheap fit at that) that I face the windshield toward the sun, and when I get in the car it is really quite surprising how cool it is. There may be more fashionable solutions, but there are not any that are more effective. You will notice that no one uses cloth painted silver on one side and black on the other for house and building insulation, but they DO use a similar mirrored bubble wrap for that very purpose. (I have seen it done in Hawaii in a roof space.)
My main desire is protection of my interior surfaces from the sun's rays beating on them, but the thermal insulation is also an appreciated bonus.