fwiw, there is a 2" electrical conduit fitting that will screw right into the bung hole. I have had no problem with my 2" smoke stack.
I have had problems with my metal pipe nipples and caps - will nead to work on them. They rusted so bad I could not open them (cap rusted to nipple). I am not sure if I will just wire brush them and periodically coat them in oil, go to brass or galvanized. Right now they are simply a pita.
On the other hand, thinking about replacing them completely with a bbq guru.