I built this a couple years back. I don't have much info on it, but the pics might help. I went to the electrical department and made it work the way I wanted it too. I believe it's a gray coupler with a gray reducer glued to the inside of it and then I fit a white pvc coupler over the smaller gray with another reducer. Again it's been a few years and I never took a shot of the components that went into it.
The longer piece actually slides down from the top of theeewhole setup and once I go the water level right I glued it down.