Uses a Vu Meter as a tiny percussive hammer.
Checkout this video; rB5UPjEjBlM of me assembling a vu-perc module, it will help you assemble yours.
Solder small components (NOT jack sockets, switch and potmeter).
Put all big components (jack sockets, switch and potmeter) in the circuitboard and mount them with the nuts to the front panel, after this you can solder them to the PCB.
Making the vu meter, remove the vu meter enclosure with a cutter, and carefully cut a peace of the side to make it smaller (see images below).
Look at the images below and improvise.
There is shrink tube around the copper flap, this helps protect the vu meter hammer from not connecting to GND via the copper flap.
Wires connect as shown. I spray painted white paint over the piezo element so it cannot oxidise.
On the end of the copper flap there is dampening attached, this will remove high frequencies.
There are also more then one extra copper strip soldered to the back on the copper flat. This can result in a snare like ratle when the metals strips vibrate and hit each-other.
A spring is soldered to the the end of the copper flap, the sound in generates is much different from
the hitting of a copper strip.
The spring is made of a material that cannot stick to solder, so its not easy to make it stick to it, you
can use a pin from a LED or resistor and use that to hold it in place with excessive solder.
To prevent the vu-meter hammer from connecting to the spring you can slide a peace of sleeve from the wires over the end of it.
You can also use different vu-meters, most will work.