You can shoehorn a lot of stuff into that size, here is my daughters CPO, it's 29x13 and has 2x spinners, 1x trackball, 6x buttons per side, 2x led JLF joysticks (the picture has MagStiks), and 2x player select buttons. there are leds in everything, and Angle eyes under the spinners. I could even put a flightstick in the middle if I moved some stuff around, but that would be really crowded. It doesn't feel crowded at all right now, even with all those controls in it.

Just use the technical drawings for the devices you plan on using (scale them to the proper sizes), for instance, that trackball is huge on two corners, but small on the others, using the technical drawings will help avoid making mistakes by misjudging how big things are on the bottom (eg. joysticks take up 4 times the space below the panel).