Welcome to the forum Capt Q.
I built a shed for my CS55 and used the same venting as Brewbek. My vent pipe sits 3-1/2 inches above my smoker vent and I've had no problems with this set up. I have noticed on cold days that condensation will drip from the vent pipe but it does not go into the smoker. I do get some smoke inside the shed but that smoke is leaking from around the smoker door and not from the vent. The leaking is almost gone now that my smoker is well seasoned.
Good luck and enjoy your cookshack.