Most heat tubes are for pusher fans. They have a collection plenum in the front of the radiator guard that funnels the air into a tube. I have made them before using sheet metal and a 8" duct take off then to 8" flex duct. Tape the ends well to keep the fiberglass off of you. They work well , retains engine heat , and warms the operator. You can even put a blanket over the operating platform let the duct blow under it to keep warm. You know , with my survival skills I might be able to make it in the great North. These heat tubes , you would be surprised how much heat you get.