A simple one string "diddley bo" lap slide guitar made from an old piece of wood a couple of nails, a Piezo Transducer 273-073 from Radio Shack, a 1/4 mono input plug, a piece of wood for the bridge, a rubber band, a small tin can, a roll of tape and one old guitar string.
The Piezo Transducer 273-073 cost about $2 at Radio Shack, I also got the 1/4 mono input plug and extra wire at Radio Shack. . . everything else I found around the house.
The tin can originally housed cough drops, but you can use a tuna can or snuff can or any number similar of items. I removed the Piezo Transducer from it's plastic shell and tape it into the tin can. I place the guitar bridge (small piece of wood) on top of this. You can tune the thing by moving the bridge up and down the neck of the guitar

Nothing about this is original, it's just my version of a basic Diddley Bo