As for the control panel -- good plan but if you're not going to be adding anything else to that I'd lop off a couple of inches, especially for a desktop controller. Besides you can always make different ones for different needs in the future.
It is going to be made to fit on the slide out keyboard tray. I had realized that I could squeeze 2 players worth of controls in that space. But I don't want to be buddy-buddy with whomever I am playing.
Maybe ditch the P2 credit button
I use it in mamewah and for kicking my wifes behind in frogger.
Here is a pic of my Computer desk. I still haven't decided on any kind of artwork or paint for it but I am going to get it to match the blue glass.
I always see these pinball buttons on peoples cabinets. What software are you playing pinball with? All the pinball titles in Mame are broken in one way or another (usually they have graphics layer priority issues).
Thats why I didn't include them I just don't play those games or have any other need for buttons on the side.