Hi teeftoo.
I suggest yoy to use not more than 4 buttons by player (snk layout)
If you observe my panel, I use of small buttons for the administration options, is better, saves much space in the CPO.
My panel only measures 56cm of wide and 33 of bottom, a little more than your panel.I think is not a Frankenpanel

However, the 4 way stick and the virtual insert coin button,you can save them to put on the CPO.
You can use a small button down the panle for insert coin, or maybe, inside the coin door, you can put a switch
And....don't forget to put on the sides the pinball buttons.
A greeting.