The CST L-Trac trackball is a wonderfully customizable device, thanks to its two auxiliary button ports. I've seen foot-controlled switches designed to work with the CST, but some of us would prefer to have the switch mounted for use by the trackball hand.
I don't know who did it first, but I got the idea from reddit user ripster55 to use Cherry MX-style keyboard switches for my aux buttons, as seen here: http://imgur.com/a/bhicE
I designed this housing to fit the curves of the CST L-Trac and put the switch in easy reach of the right-hand thumb. This was my third iteration, and allows for two switches to be installed, including enlarged wiring ports for doubling up the wiring. It also includes some almost-certainly-unnecessary internal bracing. It didn't feel at all springy or flexy without it, but I tend to enjoy overbuilding, so there it is.
I used zigzag infil, and it was effortless to remove. I recommend printing this with the trackball-side down for long print lines, and a nicely-finished outer edge. Unfortunately, this direction defaults in some support material in the outer edge of the switch inset. It doesn't seem to have affected the fit at all, though.
The CST does not include any mounting options. You'll need to either glue this to the side of the trackball, or some up with your own ingenious mounting strategy. I have mounted mine with silicone caulk (the exact same stuff I make for brewing Oogoo). It's holding up well.