Okay. The nube bites. I've never done natural caverns before, so this will be a good learning exercise. I think the key here is that planning time is not included in the four-hour limit. Perfect. If I plan the hell out of it, I should be able to get it done in the alotted time. I also need to keep the map fairly modest...
Here's a quick bubble design that I whipped up over lunch. I did it up in Microsoft Visio (it's quick and easy, and all I have to play with during my day-job).