As I explained in my reply to your PM, an electret mic has a fairly low impedance so the Leak2 circuit has a fairly low input impedance. But a piezo transducer is a fairly high impedance so you neede a preamp with a high input impedance like used on a guitar contact mic, a drum contact mic or a car engine sounds contact mic. The Makezine video I posted shows the schematic of a suitable jfet preamp and Google shows many preamp circuits for a piezo contact mic usually using a Jfet.