My very first smoker was an old porcelain & steel fridge. We just put a smsll CI habatchi in the bottom with a few hsndfulls of charcoal and used it to smoke mullet & chicken; very low heat, probably 200* or less. One caution to take into account is you will probably only have the one big door and don't want to open it much. I think I might be tempted to design a firebox that would give me a long burn and maybe include a 1/4"-3/8" heat diffuser plate about 6"-10" above the fire.
Got a bunch of cooking toys and a custom metal fabrication shop where I spend my time building all sorts of smokers & outdoor cooking gear.