The stresses are the least of my concerns atm. First I want to get the model right.

As you see the first model has a standard tubesheet in between two flanges. As I've picked tubesheet 'configuration e' with a flange on 1 side and a channel on the other one I want to get rid of 1 of the 2 flanges in my model.
That should be model 2. I've modelled a piece of straight pipe to reflect the channel, but I have a feeling that's not the way to go.
So how do I correctly model 'configuration e' with a channel on 1 side of my tubesheet and a flange on the other?