I designed this unit off a modular shelf unit I saw some time ago. I used gears instead of tubes and designed 3 possible shelf sizes to fit however is needed into the gears. Each gear can be added or removed separably for reconfiguration of the unit. The units are 1 foot in depth and internal diameter but could be any size, even mixed sizes, because they are gears, they can be fitted more solidly than tubes. You could do the gears out of metal and the shelves in wood or reverse.