Since I have not posted any pictures lately, here is a screenshot from Alibre Design showing the "triple button" that is used for the middle mouse button. This was milled out using my Zenbot CNC router. The low areas with holes in them are where I have 22 gauge nickel metal contacts with wires soldered to their backs located. The holes are there to allow the wire to exit the back. At this point, the wire is the only thing that holds the metal contact inside the shell, but eventually that will be secured with an adhesive. For a size reference, the square area in the middle is 11.5 mm wide. Each smaller touch pad area is 5.5 mm tall. The dividers between the buttons provide tactile position feedback of your finger and are 1.5 mm tall.
You may also notice that my screenshot is looks different as I have now upgraded to Alibre Design 2012. They had one of those killer end-of-year upgrade offers I just couldn't pass up :p.