I'm planning one, but I don't intend to build it until next year. I plan to make a plexy enclosed booth....the size of a phone booth. With colored spots inside the chamber to give a fogger the look of poisonous green gas. Controlled by a simple prop controller to trigger lights, sound, and fog effects.
We must have hit the button at close to the same time. For mine, imagine a booth like Terra's...but with a metal simulated frame....and large rivets, chains, and locks. (Cut out circular pieces of wood or foam and paint metalic).
I've also been toying with the idea of a large water tank. The two may become mixed and build as a large gas chamber. Using cement sono tubes or something similar, painted to look like a big brass chamber....with a portal type window. Then, putting a person on camera to create a video so it can be replicated by others.