I agree with the above comments, but one thing you haven't mentioned..... is the trackball a 3" or a 2.25"?
If it's a 3" then consider changing to a 2.25"..... then you can push things together a little bit more to give the extra room for resting hands etc. That is exactly what I had to do with my panel, but a 2.25" trackball is fine, believe me.
In fact, you're design is almost exactly like mine. The only difference is you have extra buttons in the top left/right of the panel. I avoided extra emulation buttons to make the panel look less cluttered. My panel is 605 mm wide by 210 mm deep, and I still got an Oscar Model 3 spinner above my Ultimarc 2.25" trackball unit in that space. My joystick/button configuration is exact same as yours.