... the shift/shazam key...
This is a subject that was kind of unclear to me. I thought that I would need at least 1 extra button for the shift/shazzam key. The way it is mapped out by default on the KeyWiz (of which I own), it seems like it's wired for seperate buttons for the Pause, Exit buttons, and the shift + joystick is used for special purpose like F3. Am I just confused?
I'd wire the shift/shazzam ("sh/sh") key to it's own button. Then you could assign (in the encoder board) sh/sh + player1 button1 to send "p" for pause, sh/sh + player1 button 6 to send "esc" to exit, etc. You can assign any output for any button, and any (different) output for that button + the sh/sh button combo.
Some say wire a normal button as the sh/sh key; IMO you can run into problems with this. The biggest being you exit in the middle of a 2 player game because you and your friend accidentally pressed the sh/sh button and the button that is the sh/sh exit button at the same time.
The keywiz is a little different than the ipac in that the keywiz shazzam key is a separate key, while the ipac any of its keys can also be the shift key. But nothing's wrong with wiring a button to two keys, so you could wire a button as both the shazzam key and a normal key.
IMO, nothing's wrong with having those buttons for pause, another tab, etc, although you might want to move them off the top of the CP, or only do one button (the shazzam key).
4ways are often wired the same as player 1 8way.
I'd put player 1's buttons as trackball buttons, but you can do player 2's if you want. (You might need a ctrlr default.ini and mouse.ini file so the TB buttons are assigned to the correct player in the correct games.)
That spinner might be in a hard to use place. Are you a lefty-spinner or a righty-spinner? Which button(s) would you use on a spinner game? Is the player 1 8way or buttons in the way?
I don't play golf or bowling games, so I wouldn't worry about the 4way stick. I would worry, however about player 2's 8way stick. The way I play, I might hit my fingers when I do a heavy roll right or (especially) right & up.
1/2" high quality plywood would be okay if you didn't do the trackball, 4way, and spinner. With them, especially the TB and 4way lined up in the middle of the longest dimension, might weeken the wood enough to brake within a year of only medium play.
Also been mentioned before, but are you going to have wood sides underneth the CP top? That would take ~3/4" off around the sides and top and bottom, cramming you even more.

my $0.02