Oh, yeah. I’ve been meaning to build an electric ukulele. I’ve built some handmade electric guitars, slide guitars and similar. Mine are rudimentary, more akin to a cigar box guitar (if not less complex).
I've built half a dozen similar box guitars, it's such a fun little thing to build.
Do I see a piezo disk under the bridge in the 3rd instrument? Do you use some kind of preamp with it?
I have been recently experimenting with different kinds of piezo pickups [0] and preamp electronics for them. I've figured out a pretty nice JFET based circuit for the preamp and ordered tiny 13x13mm PCBs with tiny SMT components assembled and it works pretty well (but needs a second revision). They mount directly on the volume potentiometer and fit in a small space.
The one thing I haven't figured out yet is grounding the electronics. In a typical electric guitar you ground the electronics by touching the (grounded) strings, but that doesn't work very well (at all) with slide guitar when your left hand has got a bottle neck slide on it (made of glass or ceramic which is an insulator).
Drop a message below if you want to geek out more about home made guitars and/or related electronics. Depending on your location I could also send some preamp PCBs your way.
Happy to stay in touch. I haven't build one of these in a long time. My website and email address are in my bio here on HN.
To answer your question, yes, that's a piezo pickup, if I remember correctly. I don't think I had any kind of pre-amp. I wired it to a 1/8" stereo plug and then connected that to a very small portable guitar amp. It was all very rudimentary. A very distorted sound. I don't remember grounding to be a problem but I played the slide version of this with a slide made from a galvanized pipe, rather than glass.
Examples: https://photos.google.com/share/AF1QipPmHtIa9vAm8wNPeTXP4MoZ...