Piezo buzzers...

I've seen piezos driven by a transistor/inductor combination with the piezo in place of the catch diode. The inductive kick of the coil produces the higher voltage needed while the capacitance and loading of the piezo, prevents the transistor from dying by forming a resonant tank circuit.

A audio impedance transformer is another common option - an 8R to 1k unit is perfect.

EDIT:
I like audioguru's bottle top idea. If you drill a 5mm hole in the bottle top and enclosure and glue them all together it'll be perfect.

Lots of good ideas to try though it may not be in the next few days.

Thanks everyone for your replies.