My first CP wasn't specifically intended for use with a laptop -- I actually built it to see how all this stuff went together and to try out MAME using real controls for once -- but it would have worked fine. I built it out of scraps I had around the workshop, and it was 13.5 x 4.5 x 4. It was actually too small, though, because I didn't have enough room to mount the IPAC without it sticking out the front. I also never put a front, back, or bottom on it. But it worked great, and it inspired me to build a bigger, better one as a second test. (This one has a trackball and a spinner and fewer buttons.)
I tend to play the classics, so I don't need 6 buttons. If I were building a small, portable CP, I'd have an 8-way joystick, 3 player buttons, 3 admin buttons (P1 coin, P1 start, ESC), and a spinner. (I'd like a trackball, but it would kind of ruin the "small" part.)
Anyway, I've attached pics of my first CP -- you can see what I mean about fitting all that stuff in such a small space.
-- Chris