Robert Groves, I made the air dam from aluminum sheet. But you can use just about any material. You want to create a low pressure (partial vacuum to help get the hot air out). Since you do not have a pitts muffler right under the carb you might be able to get by without the ducted air. With the pitts muffler the duct air is required. How do I know? I was flying and had engine failure, but landed ok. It turned out one of the two ducts had come loose and was inop. Once repaired, it ran ok, as before.