The diaphragm of this engine is made from a balloon (just an ordinary one, nothing special!). It's reinforced with a piece of inner tube in the centre First of all cut the neck off a balloon and stretch the balloon end over the top of a can. Then cut a piece of inner-tube rubber about 1cm square and glue it in the centre of the balloon.
Once the glue is dry, you can use a drawing pin to pierce a hole in the centre of the diaphragm for the displacer wire. Leave the pin in the hole until your ready to fit the displacer later.