First rule.
Never, ever bolt up a larger panel to a classic style cabinet, it has never looked good, people will laugh at it, and it is easy to make the cabinet into a tip over hazard by doing so.
I say skip the trackball completely and just drill out 2 more buttons per player on that panel, or you could even drill it out to 6, although the spacing would be slightly off.
I would not suggest a diamond pattern for 4 buttons in any case, that was only ever used on a handful of games. A 4 button layout should go generally across if for no other reason than the fact that hundreds of games show that pattern in the demo screen.