In response to this thread, a rough and ready description on how to make using an oil drum 
You will need:
45 gallon oil drum
Wood
Sacrifical Charcoal
Lighter
Time
End A (visible) has slits cut in it; End B has its circumference cut out, almost up to the rim. This will allow for the cut out bit (referred to henceforth as Lid) to be replaced and seal the End B once again.
Start with End B facing up. Fill with wood and add some sacrifical charcoal near the top to help start the burn going.
Set alight (bit of trangia meths will help).
Once properly burning/smoking replace and wedge in that Lid.
You need help for this bit - flip the whole thing upside down, End A upwards and place 3 equal bits of wood underneath to maintain airflow.
Let it smoke away - white is water vapour, yellow is tar, blue is almost there, clear/heat waves is what you're aiming for and for this size of burn can take 4-6 hours.
When the smoke clears, kick out the supporting wood and bank up the sides and top with earth to seal everything in - airtight, or as close to it is what you're aiming for.
Allow to cool.
Hey Presto - charcoal
http://www.bushcraftuk.com/forum/showthread.php?t=55636

You will need:
45 gallon oil drum
Wood
Sacrifical Charcoal
Lighter
Time
End A (visible) has slits cut in it; End B has its circumference cut out, almost up to the rim. This will allow for the cut out bit (referred to henceforth as Lid) to be replaced and seal the End B once again.

Start with End B facing up. Fill with wood and add some sacrifical charcoal near the top to help start the burn going.
Set alight (bit of trangia meths will help).
Once properly burning/smoking replace and wedge in that Lid.
You need help for this bit - flip the whole thing upside down, End A upwards and place 3 equal bits of wood underneath to maintain airflow.
Let it smoke away - white is water vapour, yellow is tar, blue is almost there, clear/heat waves is what you're aiming for and for this size of burn can take 4-6 hours.

When the smoke clears, kick out the supporting wood and bank up the sides and top with earth to seal everything in - airtight, or as close to it is what you're aiming for.
Allow to cool.
Hey Presto - charcoal

http://www.bushcraftuk.com/forum/showthread.php?t=55636