2x4 or 1x4 board, approximately 2 feet long
7 inch rubber bands (available at Staples)
Wrap the rubber bands lengthwise around the board.
Blow up the balloon. Big is OK!
Pull up on the rubber bands and push the balloon underneath them so it is trapped between them and the board. It can be in the center or towards one end.
Slide a pencil between the balloon and the rubber bands, one pencil on each side. The rubber bands should only touch the balloon on its top, not after the go over the pencil.
Pluck the "strings"gently.
You can tune the zither by pulling the rubber bands more tightly to the bottom of the board.
You can increase the sound (amplification) by leaning a styrofoam cooler or cardboard box against the strings.