Great tips on how to install an internal piezo pick up. This link takes you to a great 2 part article about how and where to put a piezo pickup in a cigar box guitar, which is pretty similar. Here’s another article on how to wire one up. Here’s an instructable on it too. In the following video I particularly like the tip on how to easily get an open jack (not an endpin jack) installed.
And here’s how to solder a piezo. Wish I had watched this first before ruining the piezo I pulled out of an old fire alarm/smoke detector.
And here‘s how to install one according to this guy’s blog.
And the same guy put one in a drum box/acoustic stompbox.