Guys, IMHO we're dancing around symptoms of the problem. Heat rises, and that's the very basis of a good air flow/draw which results in a nice hot clean burning fire. Enter:
> extend smokestack to grate level,
IMHO, this is the culprit. Your cook should've been somewhere around 10 to 12 hours, not 16+. In this going through perhaps 2 bags of charcoal, at the most, on a small cooker like this should be fine.
Can you easily remove that one modification and see how it goes? The basket looks great to me.