Flunky, have you considered a button between the joystick and spinner?
It could be used as reverse in defender or stargate, but even more helpful for those who spin with their right hand. A fire button on the left is what they'd want for games like tempest, arkanoid, etc.
Just a thought.
I have considered what to do about the spinner. I've got the spinner and a few buttons mounted on a piece of hardboard to kind of test the layout. I feel much more comfortable operating the spinner with my right hand, but it also feels weird firing with my left hand. 
I added a button on my drawing between the spinner and the joystick. I also moved the 1p/2p buttons to the front and moved the two admin buttons to the left side.
Any opinions on this new layout? What do you guys think of having the 1p/2p buttons on the front?
Still looks good, but I don't know about the buttons on the front. I think you could still safely and easily get those back onto the CP. As long as you use buttons that are labeled as 1P and 2P, no one will confuse them.
Another consideration would be to bump the buttons and spinner up a bit to give your palms somewhere to rest.
What are you using the volcano buttons for? Also, if you keep them on that side, I'd line them up horizontally as previously drawn. The way they are in this drawing doesn't look quite right to me.
Are you using a coin door?
(I guess you ruled out Asteroids, etc.?)
The player select buttons I have are labeled, so there shouldn't be any confusion if I decided to put them back topside.
The CP is only 5.5" deep, so the heel of my hand doesn't rest on the CP. I'm making a mock-up, and I'll see how it all feels before I commit.
The volcano buttons are admins for exit and pause.
I did get the small coin door from Twisted quarter, so I can credit up with coins. Not visible in the Sketchup image is another button (I'm considering) just under the front of the CP. It won't be visible from any normal vantage point, but it will be used to credit up without having to open the coin door or go to the swear jar.

I did rule out Asteroids.

Actually, my thinking is that I might only load vertical games on this machine. I have another son coming up behind the one this is being built for. My wife told me, I'll need to make a horizontal cabinet for him... so I'd like to work the CP differently on the next one.