Using scraps of pallet wood and some tiny speaker components I managed to piece together this little prototype – it uses a 2x 3w amplifier and is good for about 2 weeks of use on 4AA batteries.
Possibly due to the nature of the softwood used for pallets it actually sounds a bit bigger than it is – its no bass monster obviously but its not tinny either, and 3w is more than enough for such a small speaker.