IMO, the double-barrel smoker is quite an undertaking for something that's just sheet metal thin and only has so much life to it before burn or rust out....even if there's no welding required. It looks like a pretty big fuel hog to me.
If you're thinking about an offset made with drums, the Char-griller offsets and others are CHEAP, and some of them are pretty big. Tell you what though, you're gonna go to a WHOLE LOT of trouble getting an offset made out of a drum to cook anywhere near as good as a BDS clone or UDS. Heck, I don't even have a firebox on my char-griller...because I have a UDS and a wsm. However.... "It's the cook, not the cooker."