The MIDI Stomping Pads are assembled with 3D printed squares. They print with no supports. Two squares will be needed to assemble each stomping pad.
The cpxMidiStompingPad.stl can be downloaded directly here or from Thingiverse.
The 3D printed square has a raised section in the middle to place the copper foil. There are also slots to place the springs in the corners and a slot to run the wire to the Circuit Playground Express.
The Circuit Playground Express can be housed in this 3D printed case by the Ruiz Brothers, found in this Learn Guide.
Alternatively, you can use the Adafruit Circuit Playground Enclosure sold in the Adafruit store.