I think that you could build it your self. Just figure out where you want your grates, and drill the holes. Drill and install at least4 1" long 14 bolts to hold each grill and you're set. As far as vents, you have several options from cutting a door with a jig saw to just drilling holes. I would cut a door for the fire, as reloading the tray will be tricky as it is so low in the cooker. Just use brass cabinet hinges to attach the door, and adjust flow by opening or closing the door. A 55 gallon drum is about 22" in diameter, which is the same as the weber kettles. You might have to measure your specific drum to be sure. As far as the drum, just ask around for a clean unlined can. We have several here we use for untreated water, but I'm a long way from Kali. Oh yeah, you can use the bung for exhaust, or just prop the top up a little. I would be glad to help you build one!
All you would need is a power drill, jig saw, 8 or 12 1" long 1/4" bolts (or larger), 2 or 3 grills, a pan for charcoal, and a couple of brass cabinet hinges. Including the drum, you should be less than $50.00, with most of that in your cooking grates. Probably take you less than an hour or two to put it together.
