I think your best bet is to use a material that is not heat conductive. Aluminum is very temperature sensitive, and ATTRACTS heat just as fast as it draws heat away. When you park hot, the plate will draw heat towards the carb if the carb is cooler than the engine when you stop. Not what you want. There is some "heat sink" effect with the large surface area, but I'd bet your plexiglass shield is better than aluminum.