As you've noted - it's unlikely there'll be much spare room inside the enclosure, so that may not be a viable route.
You could perhaps make a small add-on speaker module that connected to the Zune's headphone socket (I'm not familiar with the Zune, so don't know the layout). Alternatively, you could try making a new (bigger) enclosure, but I suspect it would be difficult.
Finally, I'd be surprised if you could get much quality of sound with a tiny source. If the goal is some level of portability, could you just take a small pair of flat panel speakers with you? If the goal is playing gangsta rap at full volume on the back of a bus, then I can help, but only with a baseball bat

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