This decorative bottom plate designed by Katie Doodles, and made out of FR4 (PCB material).
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/
(Keycaps, switches, Planck PCB and switch plate not included)