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). 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)