About 3 inches wide, then it's pretty much smooth.

Here's a render of the latest revision. It's little bigger (effective diameter of 8ish inches), and uses more machining resources. to shave weight off.

Once I hit a revision that i feel comfortable putting on a robot, I'll be hosting the CAD for anyone that wants it, with only one rule: if you make it or modify it, you also have to put the CAD up. It's time to bring Open Source to robotics, folks.