I agree that the box shouldn't get any larger. The speaker I have is just too large (it's a 5"). I can get a generic 4" speaker from Jaycar for $5. It will easily fit into the current planned dimensions. I was just at Jaycar today getting some parts for my current pedal and amp builds. I should have picked up one of the speakers.

The piezo I tested is a 3cm disk. It probably wasn't a totally fair test though, as the box was a bit dinky, making it hard to stomp with my foot. A lot of people use piezo mics in these, so it should work. The box Brendan built looks too small to house a speaker, so I suspect it has a piezo inside.