Here's another Arduino based project for building custom midi controllers that could be of interest.
This ones seems very user friendly with a web based configurator:
OpenDeck is an open-source platform for building custom MIDI controllers compatible with any MIDI software and hardware on any OS.
Using OpenDeck building of MIDI controllers is simpler than ever.
Instead of spending time on choosing the right electronics, writing code, and solving lots of unnecessary problems, simply connect components of your MIDI controller into OpenDeck board and you’re done.
Additional board configuration can be done online – no installation of extra software is necessary!
Class-compliant USB MIDI
Easy configuration using Web interface
No drivers or extra software needed
Compatible with all MIDI software and hardware
Works on Windows, Mac, Linux, iOS and Android
Single color and RGB LEDs (with PWM)
Official OpenDeck board
Arduino Pro Micro