I wanted to make it easy to plug into the Arduino Nano to update the code, even once the project was all buttoned up. At one point I was even considering enclosing the whole back of the pack with some kind of plastic sheet, so it was especially important to break out an interface early on (even though in the end, I didn't go through with the enclosure).

So I plugged a USB-B panel mount adapter into the Arduino, and ran it up above where the neutrino wand would have mounted. Incidentally, for this build, I decided not to do the wand. My daughter is a bit too young to understand the concept, and frankly, I was worried she would hit somebody with the thing.