I glued the black layer of craft foam to the stiffener. I then glued the red layer of craft foam to the opposite side of the stiffener. This created a bottom which was black on one side, red on the other, and was plenty stiff. I did the same for the blue material. I then glued the 20mm foam cutout pattern to the backing, one for SAE, the other for Metric. Here is the final product, with all the labels beneath each wrench.