I built a midi controller. Rather, I repurposed a Teensy-based virtual analogue synth I made ages back that never worked quite right. I had taken the amp/speaker out to use somewhere else, and it was just sitting in a box. I realized I could just use the Control Surface library (https://github.com/tttapa/Control-Surface) and change it to a midi controller in no time flat, so I did. Now just need to get some time to use it to play with VCV Rack.