building a smoker / bbq

hi there,
about a year and a half ago a made a stainless steel barrel smoker. It works good, but is to small now and has a problem keepin a low temp. (lower than
80 C. the fire will need a lot af attention). I'm thinking of building a new one. this time a bigger one Off set model. the question is, when using a 800 (or 1000) liter barrel (180 cm long, 80 cm diameter) how big should the firebox be. I can get a 4mm stainless steel box(66 cm, 85cm, 61cm) for about 120 $. but it seems a bit to big
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