Stack size on Stumps Smoker Baby?

Does any know the diameter of the stack size to the Stumps Smoker The Baby? Thinking about fabricating something for it even though its not here yet. My WSM top vents are stuck open from different "tar" and in the right situations when raining ill end up with 2-3" in the bowl. Don't want anything stopping me from my addiction.

Saturday cook: Smoker heating up currently at 189 for a 9lbs Boston butt and a pork belly.