Andy,
I think the links Rasputin posted and Indianna's link to Rockler should give you all the information you need, I think the easiest would be to build one similar to the picture Richard posted to avoid the need to build a sunken pit. If you plan to use this outdoors or are considering using a roundhouse then things become a bit stickier since you will probable want to index the wheel and motorize it. I would suggest building an inexpensive model using a lazysusan and once you have a feel for the size and weight requirements upgrade it using plywood and hardware from Rockler. I don't know where you are located so I have no idea what resources are available or how comfortable you are in scratchbuilding, keep this thread alive and I am sure we can work something out!
Gepetto
