About the control panel, I'm trying to build one that can play Street Fighter 2, Robotron, Araknoid, Galaga, Mrs. Pac-Man, and Crystal Castles. If the CP is good for all of those, I'm happy.
Good game choices. Let's examine the control requirements. First we will look at one player action only, and then what would be required to make it a two player panel.
Robotron requires two 8-way sticks, and it doesn't feel right unless they are leaf. So now we have two 8-way sticks.
Street Fighter 2 adds 6 buttons to the right of the first 8-way stick. Mrs. Pac adds a 4-way to the left of the first 8-way (sharing the buttons that the 8-way uses).
Galaga adds nothing.
Crystal Castles adds a trackball, which I would place on the far right.
Arkanoid will play fine with a trackball, but is better with a spinner.
So that would leave you with this minimum layout.
P1 P2
4WAY 8WAY 123 8WAY TRACKBALL
456
P1 and P2 are start buttons. Everything else seems self explanatory.
That layout will fit on a 24" panel, but just barely. Right now I have a 22" panel, and have that layout minus the second 8-way. 2 more inches and it would have fit easily.
Adding a second player adds about 5-10" width to that layout, because you need not only the room for the second set of 6 buttons, but you also need a few inches of breathing room so player 2's joystick hand isn't bapping your button hand. Sticking the trackball between player 1 buttons and player 2 stick removes the need for an extra space, but when combined with a second 4-way (for player 2), it places the two 8-ways a lot further apart than what would be optimal for Robotron (which really shouldn't affect you, unless you have been practicing on a dedicated machine for 20 years).
I am not a big fan of adding controls that are above or below other controls in the layout, because they often get in the way of each other. But straight across works great.