I went to the local Computer City and found a large trackball for $20. I brought it home, and took it apart so that I could think of how to mount it. I came up with the idea of simply removing the top of the assembly, and putting it through a large hole in the control panel. Here you see a picture of CPv2 with the hole cut.

This is a picture of CPv1, but the installation is the same. You can see that the hole is somewhat irregular. That doesn't matter too much, as later on I will be installing a faceplate. Ignore the hole for the joystick--on this control panel prototype (CPv1), I didn't mount it properly.

Here you can see how it looks when mounted (with the faceplate removed). It is screwed into the control panel from underneath, and through the plastic of the trackball. It is also attached by a screw from the top into the edge of the control panel. All three screws have to be carefully adjusted because they have tendency to distort the shape of the trackball frame. If it's distorted too much, the trackball doesn't work well.

And here it is from the back. It's a little hard to see much. The top of the trackball's frame (if you were looking at the control panel) is flush with the top of control panel. The bottom of the frame, however, is underneath the control panel. This is how I have a surface to attach screws from underneath.

Since this machine isn't going on site :) the trackball doesn't have to be secured in. That's good, because the faceplate that I ordered from Happs is a little bit too big. I have about a 1/16" gap around the ball. Notice that I'm missing a bolt in this picture :)

And here's the final result. About half of the ball is exposed--much more than is exposed on real arcade games. However, for $20 this trackball works extremely well. I use a default mouse driver, and I adjust mouse settings for games individually. Centipede plays great! Here you can also see the mouse button (red). Normally I don't use it for playing games. Rather, I use it to navigate through Windows.