Here's a very small album I put together a few years ago that shows both the prototypes and the final mechanism we had.
1276 Prototyping Album
There are very few things you cannot prototype with 2x4s, plywood, PVC Pipe, 3/8 threaded rod & nuts, drills, and polycord. You can drive basically anything by chucking it into a cordless drill, just be sure to have enough batteries!