The result is a variable resistor that can be triggered with just a little pressure from hands or feet. Resistance ranges from 0 to about 18k ohms, and it's very sensitive to slight changes in pressure. (With no pressure, resistance is infinite.)
With an Arduino, the clamshell's big jumps in resistance can work as an on/off switch-- perfect for foot-powered control! Depending on the code, you can use it as a toggle switch or a momentary switch. And as a stompbox, it's flat and quiet (unlike metal stompswitches).
clamshell CD case
adhesive-backed copper foil
wire (22 AWG)
plywood board (cardboard could also work)
10k resistor (approximately)
hole punch (or use the box cutter)
soldering work station (soldering iron, helping hands, goggles, etc.)
Step 1: The Clamshell Variable Resistor
Two holes punched on the same side of the case provide an exit for two wire leads, and prevent the case from crushing the wires.
The wire leads are soldered to two identical flag-shaped pieces of copper foil stuck to the insides of the case (on opposite sides). When the case is closed, the "tails" of foil extend up opposite sides, not touching (very important).
(I didn't take pictures while modifying the clamshell case so I'm hoping you can follow from the photos of the finished piece. I've included a suggested order of steps, later on.)