This one has the advantage of a small footprint & pot mounted on/off, volume control.
or Gittys, same size...
These can both be driven with a piezo straight in, no preamp. Though not really loud.
I built a very simple amp using just 3 componenets, an LM386 chip and 2 x 47uf electrotype caps.
Hooked up with an 8ohm speaker, 9 volt battery and a 1/4" jack socket.
I built this so I could easily test piezo pickups I make, I have not tried this as an onboard but with a little bit of adapting I think it should work, connecting a piezo to the input.
Built on stripborad I have attached some photos and a rough wiring diagram. On the stripboard cut the foil strip under the Lm386 between pins 1-6 leaving 7&8 intact and also cut between connection points of the 2 caps.
Connect all grounds and off you go.
The only disadvantage is the lack of an on/off switch but a simple spst switch could be used.
Hope this helps
all the best
Just another I forgot, you will have to substitute the in jack with your piezo.