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). 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)