Skyhigh,
My fire box for my 300 gallon grill is an old hot water heater. It's galvanized, but we haven't died from the fumes yet. I welded it low on one side, and put the flew high on the other side. It runs a pretty constant 225-250 degrees. I put a damper at the top of the pipe. Choke it back it builds up heat. Open it and it let's heat out. Simple but works. My meat grate is some expanded metal. Took half a day tops to build. It's on wheels of course. I've cooked everything from hotdogs to whole hogs in it. No shoveling hot coals. Just build a small hard wood fire in the hot water heater. I like to blend oak and hickory. Mt dew can chickens are my favorite. It's virtually indestructible. I've considered renting it out, but it's hard to find because people borrow it almost every weekend.