I built my first oil drum smoker in 1980. Different versions with several doors / lids ect. What worked for me was a firebox the full length of the drum with a baffeled opening, this way you build fire where you wanted to enter, I used direct and also indirect heat travel patterns and both worked great. In my opinion this cooker has alot of heat loss and your boxx is not big enough, what you also could do is install the stove inside the oil tank itself, and it will go up to temp. But I would build a full length wood shed out the back there fore you can have big or little fire on any end.
[I][FONT=Comic Sans MS]TwinTech Pro Double, Backwoods Fat Boy, TwinTech Hawg Kooker, FE 400, SP SPK 700, Empty check book and understanding wife...sometimes..[/FONT][/I]