Jim,

It'd probably be easier to just use the 55 gallon steel drum method*
since you can seal that up completely air tight. The K seals up well,
but it's not absolutely a 100% seal. Now, whether or not that makes a
difference, I can't tell since I've never made charcoal. My #7 will
definitely seal up well enough to kill a charcoal fire, but I don't
know if it would let in enough air to keep the wood fire going and let
the wood turn to ashes instead of charcoal.

You'd also get a lot more charcoal for your trouble.

In Mexico, they dig a big pit, fill it with mesquite, set it on fire,
cover it up except for one or two holes, then cover those to smother
the fire when it's just right. That would explain the occasional rock
in my bags of Mexquite, but at $9 for 40#, I'm not complaining.

* I think I read about that in a pryotechnics book or newsgroup. I
came up with
http://tinyurl.com/rlpx after a google.