[elhigh]

Adding a belly pan is already a good idea; if you have a belly pan down there then carefully applying a layer of foil for a radiant heat reflector could provide the kind of results you're looking for.

Just be careful you don't accidentally insulate the heat of your exhaust system inside the belly pan space.

There are clear or nearly clear window films that are supposed to be really good at reflecting heat back out of the window - clear enough that you can legally apply them to the windshield, which you certainly can't do with tinted films. You can't get them on the consumer market, they have to be applied by certified installers.