Something to add to the 55 gallon drum design. Make a sturdy metal frame for the drum so it can be layed on its side and about a foot to foot and a half off the ground. Maybe out of welded pipe. Put an exhaust pipe out of the top but route it back under the length of the barrel. Drill several quarter inch holes in the section of exhaust pipe under the barrel and cap the end of the pipe. The smoke and gases coming off the wood you are making charcoal out of are flammable. When it gets hot it will act like a blow torch coming out of the holes. You will use less wood for your fire. You can also stack cinder blocks around the barrel increasing efficiency as well. Just remember to leave a space to feed your fire as needed.