Here you can see all the wiring in place, including the power switch at the top (left in this photo), the 18650 cell holder in the middle, and the Arduino and buck converter mounted to either side of the plastic support behind the cyclotron.

As you can see, there's still room in there to add a speaker and some sound playback hardware. But as my daughter has sensory issues, I didn't think having the pack make noise would be wise. That said, the option is there to add it in the future.