DIY musical instruments are a whole different ball game. If you’re looking for a place to start, Adafruit has a guide to building your own MIDI controller using an Arduino.
The OONTZ uses a 3D-printed case, a 16-button keypad, an Arduino Leonardo, a monochrome driver and a few potentiometers. It’s actually simple to build yourself provided you can get the case printed somewhere. Just hook up a few components and uploading the software to the Arduino. When you’re done, you’ll have your own USB MIDI controller.
Mini OONTZ: 3D Printed MIDI Controller [Adafruit]