Aurora Mixer is an open source USB multichannel DJ mixer. It’s compatible with most MIDI software. The mixer has 24 programmable knobs, 2 linear channel faders, and a single a/b crossfader. At the heart of the Aurora is a ADG732 32-channel multiplexer and a PIC18LF4525. These chips are used to record the values of the knobs, sliders, and buttons.
Unfortunately, they are no longer taking orders for the mixer, but their website has all the files you’d need to make your own.