One more thing to mention. The two buttons in the front will not be useable while playing most trackball games.
But not a big deal, but you will be using player 1 or 2 buttons instead. I feel more comfortable using the player two buttons myself, which would mean remapping ALL the player 2 buttons to player 1 buttons on trackball games... This is a pain.
You might want to see what you can do about putting the buttons on the side. I don't see how, and it would make it much less pretty... but it might be useful.
btw, as for making mock CP's... I would go with pressboard. You can get it for 3 or 4 bucks for a 4x8 sheet at homedepot... and its strong enought to handle a few days. I've rebuilt my 2 player 8way control panel about 3 times before I got it right... and many times I played for a week before wishing to change it. So I would say make a functional version and let it sit for a week+ and really play test.