This decorative bottom plate designed by Katie Doodles, and made out of FR4 (PCB material). This contains cut-outs for the optional add-ons on the Rev 6.x PCB.
The Planck PCB (rev 5 or later) attaches to the bottom plate via 6 spacers + screws (screws are included with the Planck PCB). The space between the USB port and the bottom of the rubber feet (also included) is less than standard cases, so you may want to use a slimmer USB cable, or thicker rubber feet.
See more of Katie's work: http://katiedoodles.com/
(Does not include Planck PCB, switch plate, switches or keycaps)