The Kensington Expert Trackball Mouse is my favorite gaming mouse. This trackball, however, was not really designed for gaming since you only have two immediate buttons and two buttons at the top. Therefore, I decided to add additional immediate buttons.
This is a modification of my Kensington Mouse where it is augmented with a small 4-key keypad. Using a set of MX Cherry speed switches, I implemented the keypad and wired them up to an Arduino Pro Micro. The small finger now has access to four inputs instead of one.
This required a lot of dremeling, so use one of those battery-operated dremels and purchase those Dremel variety bit kits. And you'll need to use the Double Sided PCB Proto Board for the Arduino.
From testing, I recommend moving the bottom two keys to the left by about 3-7mm for it to be more ergonomic.
I designed a stand for the Mx Cherry switches, along with a base. Let me know if you have any questions.