The function button on the side of the gun was feeling pretty useless, and I really wanted to fit a joystick with a center button in that hole somehow. I was very fortunate to find a cheap 5-way nav switch that fit. However, I didn't want to run extra wires down the length of the gun. I was able to still fully use all 5 switches on one wire buy making a resistor divider network and reading in the voltage at each node.  The switch fits on a small oshpark pcb I designed.

This worked so well, and was so much cheaper and easier to build that I also decided to scrap the analog joystick and extra buttons on the main control panel for a second one of these. 10 switches on only 2 wires! This only works well in this case because the switch physically prevents you from actuating more than one switch at a time.
PCB: https://oshpark.com/shared_projects/1sKpkusr