BUILD THE BOX FRAME.
1. Attach the two frame sides (D) to the ends of the shelf assembly. Begin by measuring and marking a line 2" up from the bottom edge of each frame side. The stretcher's lower edges will fit here.
2. Position the stiffener (G) between the frame sides at the top centerpoints. Mark the stiffener's position, making sure the top of the stiffener is flush with the tops of the frame sides. Apply glue to all the joints.
3. Clamp the stretchers and stiffener in position with bar clamps. Drill two evenly spaced 1/8" pilot holes through each frame side and into the ends of the stiffener. Drive in 2" deck screws to secure the stiffener (photo B). For extra shelf support, drill pilot holes and drive a screw through the center of each frame side into the shelf.