Interesting layout, Inky613.
Since you haven't decided on the player button layout yet,
Slagcoin should be your first stop to get the button layout and spacing worked out and tested on cardboard.
Once you choose a layout, do a full mockup on cardboard, 1/4" hardboard, or similar.
Several things to look at:
1. Will there be enough room for wiring on P1 joystick?
2. You have P1/P2 start, P1/P2 coin, a Shazzaam button, and 4 other admin buttons? What are the 4 functions and what functions will you use the Shazzaam button for?
3. is there enough room to rest your wrists in front of P1/P2 joystick/buttons? Will the front edge cut into your wrists?
4. Will your left arm hit P2 joystick when you're using the 4-way?
5. The admin buttons are pretty tight in the upper left. Consider shifting them a little to the right and increasing the spacing a bit.

6. Consider moving the trackball closer to the player.

Scott