Maybe even move the trackball closer to the front of the control panel edge as well and move the 4-way joystick (assuming that is what the red circle represents) to be lower so you do not have to reach so far.
This configuration seems to save you depth space on your control panel which can be reduced now.
Now a spinner would fit nicely opposite the 4-way red circle joystick.
Maybe you could get an E-Stik from Ultimarc (which is a joystick you can set to be an 8-way, a true 4-way or even a diagonal for Q-bert, Congo Bongo, Zaxxon, etc.. games) and use this joystick instead of one of your current 8-ways .... this will free up more room since you would only need 2 joysticks now, instead of 3....
Am I basterdizing this idea too much ..... I guess I will be done now. Maybe some of my dieas make sense.
Good luck ... I am interested in seeing how this progresses...
PS: Would side pinball buttons and/or a pause button be a nice additions?

ok ... done now!