My rack is a three slot, using 90qt tubs (same length and width as a 40, but 12" tall) with space on top for a 20long tank. I did a pictorial of building it that is up on my website Grove of the Magi (link in sig).

Building is MUCH less expensive and you can customize however you wish. Those are the main reasons I don't get an RBI rack, tho Rich is Da Man!