I made mine out of approx. 1/2-inch pressboard cut with a jigsaw. Most people make irregularly shaped cams to produce a random (for a short time anyway) pattern of shaking/bumping. I used a sawtooth-designed cam with a variable speed programmer from MonsterGuts.com.
Mine, I did out of 2-layers of 1/2 inch plywood, which comes out less than an inch in width. With a 1-inch follower, it works very well. The design is pretty much up to you, I cut mine on a bandsaw, but the more eccentric, the better. Even with an eccentric cam, you will be able to tell it's a machine if you watch it long enough, the brain picks up on patterns. There are electronic ways to program your box to work in reverse and move in reasonably unpredictable ways, but of course, that adds time, complexity and expense to your prop. If people are only going to see it for a few seconds, it doesn't really matter. If it's sitting out where people can see it for a long time, it probably does.