(Dead Link Removed)

You can get rackmount rails from companies like Guitar Center, Markertek, Musicians Friend, etc. This is how we built my control room. Just slap those rack rails up and voilà, you're done.

Railing maniac
Mx. Remy Ann David

Buy pre-drilled steel rack rail online. The rest couldn't be easier.... it's a 4-sided box.

1.75 inches X number of rack spaces you want. This is a 12-space, depth is arbitrary, shown as 18" deep.

Some studios prefer wooden rails to prevent bonding the grounds of all the units. You just can't remove and replace woodscrews in the same place over and over.

If you want it to be beautiful use furniture grade plywood and veneer tape on the edges.
Good plywood isn't cheap and the end result may not be any less expensive than buying an econo-rack ready to assemble. DIY can give you custom results, but isn't always cheaper in the long run.

If you're building, I'd recommend putting the top and bottom inside the side pieces. (rather than above and below the side pieces) - see image. Let the rails (wood or steel) work for you.

Not much need to worry about bracing, since the gear securely screwed to the rails will keep the rack from shifting laterally.

If you don't like the exposed heads of the carriage bolts, you can use short wood screws to attach the rails from the inside.