Today I finished a prototype where I incorporated all the findings and conclusions I gained from my previous experiments.
A steel ball is mounted to the rotating shaft of a gear motor. The motor is controlled by the SPICA circuit. The ball is pressed against the diaphragm on a tube. The tube is closed and a microphone picks up the signal from the inside of the tube.
The signal is post processed online with the lingua device.
I also implemented a hardware reverb module but it doesn't add much to the signal quality.