End your stove pipe where it joins from the top of the barrel, do not extend it into the barrel, it will burn much more efficiently. Make a 3-inch collar from a piece of pipe one size larger than the stovepipe diameter and weld it to the top of the barrel where you will insert the stovepipe. Drill three holes equidistantly around the collar for the sheet metal screws (if you wish to lower the chimney for later storage). Weld a steel lip or other type of gasket around the top door and put a high temperature wood stove gasket on it for better air tightness, you will lose vacuum here for the chimney draft. (you don't have to do this at the bottom door, but with airflow valves it's a good idea). If you can extend the stovepipe another four feet, it will greatly improve your chimney draft and it will burn very hot, fast and efficiently. Do this and it will hum like a jet engine. Put air flow control valves on the bottom door and make it airtight and you can control how fast it burns and easily turn it off if it gets hot.