Bear with me on this, because maybe I am talking out my azz-- As I understand it, the drum "pulls" the heat out of the initially hot exhaust, allowing it to cool enough to descend instead of rise,,, so with that in mind, I would weld cooling fins on it. Granted, it would not have the asthetics (sp??) that some would like, but it would create more surface area to pull heat out of the exhaust, and if done neatly, would at least break up/disguise a crappy old barell. Someone with the right fabrication and artistic skills could make something pretty nice I bet.
Although personally, I like the idea of punding some artwork into the barell- I am envisioning skulls n demon faces looking like they are trying to break through from the inside of the barell, but I doubt I have that kind of talent.