[Tommy]

I'm going to try to be helpful without being too technical. The three major means of heat transfer in your car will be conduction, radiation and convection. Conduction occurs when something hot comes into contact with something cooler and transfers its heat to the cooler object. Your solution is to make sure nothing hot comes into direct contact with your cockpit. Radiation is mostly associated with very hot objects like exhaust pipes producing a heat you can feel from a distance. They are actually glowing light in a part of the spectrum we feel rather than see. You can block that light the same way you'd block any other light, with an opaque shield (e.g., aluminum plate).

Convection is when air is heated by contact with the heat source and then circulates to come into contact with a cooler object to which it will transfer its heat. This is what we feel when warm air from our car heater blows out of the dash. Circulating air is pretty good at transferring heat. Stationary air is not very good at transferring heat. That is why a lot of home insulation is little more than material trapping pockets of air that cannot circulate around inside the insulation.

With the science lesson complete, the bottom line is that for optimum insulation of your car you'll want to guard against all three ways heat can seep in. First, don't let hot stuff contact anything that will contact the inside of your cockpit. Second, have a radiation barrier between the cockpit and the hottest parts of your engine. A simple sheet of foil will work if it can withstand the other demands of your car. Finally, put a layer of insulating material between the radiation barrier and the cockpit. As someone said above, it could be as simple as a layer of dead air between the aluminum barrier and the firewall. A layer of foil faced bubble wrap would also work.

I hope this helped.