There used to be a ton of 3rd-party solutions for racking Apple products. Marathon Computer made a number of them, including replacement top brackets for a G3-style system: http://www.macobserver.com/news/99/june/990610/marathoncompu...

I used to have a tape measure from Marathon that was marked out in U, but I haven't seen it in years. They were a pretty cool company at the time.