I've seen bong's a foot high that work as good as my 5 footer. I've also seen them made with 3" pvc and a tiny showerhead.
There's two secrets.
1. Surface area. You want the water's surface area to be as large as possible to maximize the evaporation. In a short bong, this is done by adding something inside the tube like ping pong balls, or scotch brite pads or whatever, that does not block the airflow.
2. Waterflow. You need to keep the outlets inside the bong large enough to not slow down the flow, and small enough to keep the droplets small. If you use the ping pong balls idea or similar, you may not even need a showerhead though, because the balls distribute the water pretty well, so the flow balance issue becomes moot. Let 'er run wide open.
There's problems having it inside the case, one is the air it exhausts is humid so it's not good for all the electrical components. Another is that the bong is so small, it will only hold a few hours worth of water. A bong will use around .5 to 1 gallon of water per day, and refilling it in the case would be risky too.
There is that "project obsidian" thread that the guy built a bong into his plexi case, with the exhaust poking out the top. Just a thought.