I found what works the best is the bubble wrap type roll of insulation with the reflective aluminum coating on each side. This is available in 48" wide rolls and can be trimmed to fit your windows exactly. Once put in place, it provides privacy and reflex any heat back out the windshield.
Living in Florida we are used to dealing with lots of sun and heat. We have tried the outside covers and window films. The foil covered bubble wrap works the best and it is inexpensive. Since we seldom spend more than a few days in one location, we just roll up the insulation and store it behind the seat until the next stop. We never use our front fabric curtain when it is hot.
