Bag 13 of the Artoo set literally rounds out the top of the dome. This includes the droid's little retractable 'sensor' unit.
Here is the finished build:
Also in bag 13 is the display stand and a minifig R2.
At this point, the only thing that remains is to get the electronics wired up and running.
This will entail building a small circuit board for the array of limiting resistors as well as integrating a switch system and power source.
I will be engineering this portion over the next few days.
Stay tuned...