The bell end looks cool but really does not give you a set advantage to airflow. if you think about filling the cook chamber up with meat, any advantage you might have gained is lost. it is better to be flat in my opinion so you can cause more turbulation in the cook chamber which will also aid in a more even cook chamber temp and better smoke penetration. in other words you don't want the smoke to roll right to the top straight out of the BP Gap and then straight over to the stack and straight out....
I have a piece of pipe that's 1/2'' thick by 26'' O.D. I'm just starting to get everything together so I'm trying to get some input from some people that have built a few of them.