the boardlady once said bags that breathe and allow water to evaporate are the only ones you should use
there's these solar powered window fans you can use to create airflow within a parked car.
"ain't no big ting brudda"
If your boards are on the car roof and getting blasted by the sun you could try putting an additional empty boardbag on top of the stack. You'll need to tape the nose and possibly the tail of the boardbag down to the top board with some duct tape or maybe a little rope. The middle will be held down with whatever you're tying the boards to the roof with. I've done this in the past and the extra insulation helps keep the top board a bit cooler.
The other thing I tend to do when I'm on a road trip in the summer is to take an old board as a back-up and put that on the top of the stack, so the newer boards don't get as thrashed.
It doesn't get as hot here in NZ in summer as some places but our air is amongst the clearest in the world so the sunlight is absolutely mercyless. Many a traveller comes here not realizing this and ends up getting fried in the first half an hour. Staying out of the sun in summer is a national pass-time here.