External shade is still the best method like you said. Instead of using a small reflective cover for the front windshield only, I use a large mat/tarp made with reflective aluminum fabric that blocks most of the sun but still allow air flow. I use this for camping too as sunshade over picnic table. Highly recommended. Pick one with 85% to 90% aluminum coverage as lower numbers allow more sunlight and air, and is used mainly to shade greenhouses. When used to cover all of my car top and glass areas, the interior temp is no higher than outside temp. Of course I leave a slight crack of the windows too to allow more air flow.