Conceptually, the unit is an outer box with two vertical dividers. Nested within each cavity is a smaller box, inset by 3/8" in front, and 1/4" in in back. The front inset edges of the smaller boxes are framed with half-round trim, and the 1/4" rear inset forms a recess for a 1/4" MDF or luan back panel.

Physically, it's built slightly differenty to reduce materials (and weight). The two vertical dividers are really hollow boxes, and the top of each inner box is fake (only two inches wide).

CAD rendering (produced by MicroStation)

If you have Adobe Reader 7.0 or later, you can view this PDF version of the design. You can rotate the unit to view it from any angle, add additional lighting, or even hide various parts (to remove the outer top to see inside the vertical dividers, for example). (If you view this PDF file with an older version of Adobe Reader, you will see a blank page.)