This doesn't directly address your question, but I wouldn't do any dedicated buttons for it at all. Each control panel I make ends up with less buttons... The first one had admin buttons on it, which became a nightmare when kids used the panel -I hid those elsewhere on the cab.
The second one I made for the same cab, had 3 dedicated mouse buttons in addition to the regular 2 stick 6 button setup. It looked cluttered to me after a while. The final revision added a spinner, but I also made the trackball and spinner buttons the bottom row of the player one controls. The panel looks much cleaner and still has more functionality than my first.