Well, not hard at all!
I had a rummage in my scraps box and got all the parts required to build this for free. if you were BUYING the parts? realistically, still under a fiver!
The case is a box that came with miniature bottles of port. I took out the separators and cut the string handle.......
Now and first, I was going to go with three piezo discs because that's what I had. Pointless, so I went with two.
In the end, I used two. The biggest and smallest, passing the signal through a pot each, then to a shared jack. Superglued the discs to the inside face of the box, and its done!
Honestly, no need at all for the pots. The piezos could have easily been wired straight to the socket.
So now that the prototype works, I need to scavenge some nice hardwood!