[godot]

Interestingly I have recently purchased one of those third-party solutions (not the one you linked, one that is supposedly an even stronger fan), and these have pretty minimal effect. It helps a little bit, that's about it.

I think it's not as simple as just having more venting. When I lived in Sacramento a popular method is to leave a 0.5-1 inch gap on all the windows while parked. It actually helps quite a bit (hot air exits near the top end of the car interior), but when the car is under the sun for long enough, at best the temperature in the car may be 100F (instead of 120-140F with all windows rolled up), which is still enough to be a killer. In those sorts of temperatures I think you actually need an AC system running for it to have a real effect.

[FiatLuxDave]

Yes, those little solar fans are pretty ineffective. Cracking the windows helps but is not as good as rolling them all down. I don't know of any cases of a child dying in a car with windows rolled down but I suppose it is possible. It is mainly children being left in sealed cars which results in fatalities.

I was thinking more like venting which was designed to use the temperature difference to pull air through the car, like the designs used in desert vernacular architecture. Maybe something like a miniature Persian windcatcher, or a vent in the roof and floor which open when the car is parked to allow convection through the interior. But really anything to make the car less like a sealed hotbox.