I think i would do something like this if it was me...http://smokerbuilder.com/forums/viewtopic.php?f=46&t=166
I would do a different counter weight system if it was going to be on a trailer.
If not that then i think what EK recommended would be best. however you build it, I think it would make one heck of a UDS! or a green egg clone.
If you do put a fire box under it and feed it from the center 30% would be a good number for that since you would not be able to go for a reverse flow very easy. I would not suggest using the pipe in between method really. that does not seem to work as well since there is so much restriction and heat loss in the pipes.
You could put the square tube you have at the bottom and feed it endways into the side of the sphere at the bottom and put the baffle above the opening a few inches above the top of the inlet. that would work well i think.