This project uses switches similar to those found inside an electronic drum.
The circuit is simple: running an 5v signal though a resistor that triggers a signal when the piezo is hit. The 10K potentiometer in this model will be used to transpose ±12 chromatic steps.
The sensor is just mounted with some duct tape to make it flush with the underside of the box. The idea is that it will trigger a signal when I tap the top of the box.