Basically all there is to it is a box of some kind, we used a plastic one and a metal one, with a piezo contact mic glued inside. This is then wired to some sort of audio output, we used an RCA (Phono) out put as it's small, simple to fit and we had a few knocking about.
We then covered the Piezo and wiring in glue to give them some protection and filled the boxes up with plastic BB pellets. So armed we then threw the shaker through as many stomp boxes and effects chains as possible.
A really simple little tool to make, a passive shaker box can be a source of lots of odd, ambient or just plain crazy sounds depending on which effects you throw it through. It's also not constricted to just shaking -tap it, scratch it, drop it, shout through it, use it as a mic in front of a guitar amp - it's all good. Of course changing the contents also affects the end result - rice, marbles, fishing shot, use your imagination and all this for around a fiver!
As a taster before FM216 hits the stands there's a few samples (along with some hydrophone stuff) here.