I've used the K and K Twin Spot contact mic to pickup a variety of instruments and they make a piezo version.
The round button style like they make would likely be the best fit for a round instrument. You may need an active type with a battery to produce a strong enough usable signal if the instrument is very rigid like made out of clay. They are often used on instruments like saz, lute, oud and mandolin sized string instruments that have a smaller resonating top.