[Elgatoandaluz] has posted this guide on how to tear apart a standard optical mouse and build a custom trackball. He’s using a ping pong ball , mounted above the laser as the trackball itself, which seems like it would be a little lite, but functional. The case is scrap cardboard. We really like that you could toss this together relatively quickly and have a custom layout. He recommends using Sakasa Mouse for inverting the axes and X-control for mapping the buttons(direct download).