Here are (finally) some pictures from my build of a Tweet-A-Watt using a P3 Kill-A-Watt EZ. Since these units are readily available at Costco for around $25, it seemed like a good starting point. Please excuse the crappy iPhone photos, I didn't feel like dragging out the good camera...
With some effort, I was able to fit all the components inside the unit. You can see that the "transmit" LED had to stick out the side rather than the top
This shows the general placement of the components. The XBee breakout board itself got stuck to the main Kill-A-Watt board with two layers of double sided foam sticky tape.
Here you can see a different angle showing the placement of the super-cap and wiring.
With the sensor board placed back on top, you can see the snug fit of the components. A simple notch in the "seam" was all that was needed to let the LED poke out.
Hope these help.