I will let the pictures do the talking, due to a lack intellectual property, but I can tell you the intake and exhaust holes are smaller than beer bottle cap.
The most common BDS Clone is the 55 gallon open drum.
The mystery hole makers
the cook grate sweet spot
Modified factory air intake plugs
Dr BBQ Approved!
Now, what to do if you have an 85 gallon drum??
Well, you gotta get a couple 26.75" Weber OTG cook grates, then you can get rollin'.
Sweet spot grate specs
But, but, I wanna Mini BDS swamprb!
Well, you too can experience the wonder of the awesomeness if you can scrounge up a 15 gallon drum!
Gonna have to get you a 14" Smokey Joe cook grate.
And make yourself a charcoal basket
But, but, but...I have a 30 gallon drum with no lid swamprb...what can I do??
Well, here is what I did, because I had a rustbucket 18" Bar-B-Kettle...and I cut the lip off the bowl and tack welded it to a 30 gallon drum without a lid.
I hope this clears up any confusion. Feel free to ask me any crass questions offline. If you can't figure it out, go to http://www.bigdrumsmokers.com/
and check out the cook pics for yourself and see why I am stoked about the simplicity of the Big Drum Smoker.