I'm cutting down some cabinets from 20 inches to 12 inches to be more suitable for books. I came across one drawing somewhere showing this technique to rabbet the back of a built cabinet. It works great. I used one side I cut off the box. Leaving it long lets a helper guide it around as you rout the rabbet. You don't need to bolt the router down. I just routed a recess for it to sit in. 
