If I were mounting it, I would:
- Remove the top cover
- Cut a 2.5" hole in the CP for the ball
- Cut a 1.25" to 1.5" hole in the plexi
- Figure out how far below the CP the trackball needs to hang to just stick through the hole in the plexi without touching it
- Go to Lowes or Home Depot and buy nylon spacers this size
- Drill holes for the bolts in the CP (but not the plexi), countersunk to keep the bolt heads below plexi level
- Drill matching holes in the plastic base
- Run the bolts through the CP, through the spacers, and through the plastic base
- Fasten nuts and lockwashers to the bottom of the bolts
This is how I've mounted the two different PC trackballs that I've used, and it works fine. For best results, though, you may need to bevel the inside of the plexi to match the curve of the ball...
Oh, and this particular trackball doesn't use bearings. Although that is, in most cases, a Bad Thing, the positive side effect is that you can replace the ball with an arcade 2-14" ball or a pool ball. You'll need longer nylon spacers, though...
--Chris