I was considering this as well not long ago.
Easier to just mount everything to the underside of a table with a lip than build it into it though. A nice ITX case could be mounted underneath easily enough. Speakers mounted outward on the lip, and drill holes like a quarter inch apart so the sound can blast through. run it to a power stirp for one cable. Get extensions for major cables and run them the edge with drilled holes. Mount a usb hub on top and drill a hole for it.