I've had this one in mind for a while, having collected various small springs and seen some interesting designs on the internet for similar devices. Mine is pretty low-tech, and the piezo pickup hums an awful lot, but I'm happy with how it turned out.
The housing, as you can see, is a Cadbury biscuit tin. Mounted on top are two small springs from bi-fold closet door hardware, the tension spring left over from building the Spring Thing Mk II, a spring from an engine block, and some cut-up pieces of old guitar strings (E and G). The latter are my favourite bits on this instrument. They have a pleasing twangy metallic tone.
The pickup is mounted beneath the lid and held in place by a 2" mending strip. It's wired directly to the 1/8" output jack that I threaded through a piece of plastic (from a compact disc jewel-case) before attaching it to the tin, to prevent it from shorting-out on the metal.
