sounds good David,
the way I see it is heat rise so it probably wants to go up your coal shoot but without any air opening at the top it won't or shouldn't anyway. But if the draw on the side is not big enough heat won't go to the main chamber in hurry.
I had to add an extra chimney on my rig after I added my upright chamber... mostly because when I run both chambers or when my main chamber is really full of meat I feel need more air flow... so far it has made a great difference.
let us know how it goes once your back!
cheers