Here is one I made as a parameter knob box for controlling the GR-55, GT-100, GP-10 and others.
It uses regular potentiometers so has no control feedback from the device.
I then started on a rotary encoder version, which is capable of reading back device settings,
but this one was not completed as I couldn't get the Arduino Teensy to read all the encoders at the same time properly.
Project source code herehttps://github.com/gumtown/V-tone