Tomash Ghzegovsky developed Digital Warrior, a DIY open-source MIDI controller.

The Digital Warrior was inspired by the Abelton Push and NI Maschine.  It has a 16 voice 32-step sequencer along with many other features. It also integrates with Traktor and Abelton.

The Digital Warrior web site has a good amount of documentation including comprehensive build instructions.  You can find some additional information on it’s development on Thomash’s blog.

The project is open-source with the code available on GitHub.