This custom keypad was designed to house an Arduino Micro, 8 mechanical keyboard switches and a rotary encoder. Two 6.3 mm headphone jacks at the back can be used to attach footswitches or expression/volume pedals. At the bottom, there are recesses for 5 rubber feet (diameter: 8 mm). The included keycaps fit standard MX mechanical switches, and the used rotary encoder has a diameter of 6 mm.

I have created an additional frame to hold the Arduino in place if the latches of the bottom part are too instable and break off.

Example code for the Arduino: https://github.com/mmohn/ArduinoKeypad