I've got some I pulled out of magnetic window alarms. They're a little bigger than a bottle cap. I'll let you know about the sound once I get one wired in.
A two- stringer is also a great use of the leftover high B (string #2) and high E (string #1) tuned to G-D. very Mandolin-like sound.
One question...putting the piezo between the bridge/lid and neck, do you use hot glue or something, or just rely on the pressure to keep it in place?
This is pretty cool idea.
I got to toss a wet blanket on this one folks. What makes a detector work is a small radioactive plate, This plate is safe until you physically touch it. Piezos are cheap enough not to put yourself in harms way.