Here’s a drawing (with dimensions) for a 12U audio equipment rack.
The rack rails are available on eBay.
On the desk surface, I mounted a riser to hold my MIDI controller and computer displays in between two nightstands from Ikea. (The Rast nightstand from Ikea makes a perfect 6U equipment rack for only fifteen bucks.)
The risers are edge-glued pine, and are supported by pine 1 x 4’s. Everything is screwed together for easy disassembly at a later point in time. The Rast nightstand from Ikea is also made of edge-glued pine, so the entire desk is made of the same material. For pictures of the desk, check out my studio page.