JC Whitney and Jegs both sell some great heat blocking matting. I used the adhesive spray they also sell to stick it down. I used that in my Commando and it worked great. I lived in VA in the summer and had the same issue. I didn't need the truck and road heat adding to the misery from above while sitting in traffic!
I even went so far as to bypass my heater core in the summer due to the incidental air blowing in from the cowl vent, it just wouldn't go completely off.