If anyone out there is building your own piezo pickups, feel free to post your ideas and experiences on this thread.
I have always just mic'd my classical guitar when playing live. But I finally got sick of the feedback problems. I found that I usually needed a little more gain than I could get before feedback, and also It became too tedious having to keep the guitar-mic position in the same spot for consistency of sound.
Anyway, I did some research and built this piezo set-up.
It is a unity-gain op-amp running on two 3v batteries encased in a cheap Hammond box. The piezo element is something I scavenged from a alarm notification device and attached to the guitar body with sticky picture tape.