Keep it simple. I'm especially fond of music boxes. one of my main instruments is a celeste and I play hundreds of these types of songs. so this topic is near and dear to me.

Not sure why everyone's bringing up modes. The majority of music box songs are very simple and are usually in a major scale as music boxes generally depict traditional songs or upbeat songs for children (think "Somewhere Over the Rainbow" kind of thing). So your C major is fine. There's a [big list of music box songs][1] you can listen to. Play close attention to the 18 note and 22 note music boxes. Just by listening to these, the vast majority are major 80%+, followed by minor, other modes apart from minor probably make up less than 5%.

In a music box, usually the melody is emphasized. So start off with the melody, then harmonize it afterwards.

In order to harmonize it properly you may or may not have enough room for the chords in two octaves:<br/>
1. Invert chords to make them fit.<br/>
2. Play a subset of the chord (so instead of playing 3 notes, you'd play 2 notes or 1 note). so for example, just play the root note of the chord. or the root+fifth.<br/>
3. Try a different key. certain melodies/chords fit better in different keys for two octaves.

Another element of music boxes is to slightly arpeggiate a chord on a cadence. but very slightly almost where you can't hear the arpeggiation but it's there, it provides a cute "jumbled" kind of sound. say you're playing a chord of two notes, let's say I'm playing C-G (subset of a c major chord). so I'll play a C note, followed by G few milliseconds afterwards, so they sound like they're together but not quite. You can see an example if you look at a music box, notice that on cadences, the dots [aren't completely aligned][2].

Other suggestions: you can get a [music box maker][3] for 17 bucks if you want to test what it'll sound like after you've written your pieces. 


  [1]: https://www.musicboxattic.com/sale.html
  [2]: https://www.youtube.com/watch?v=MpDhLFHenm8
  [3]: https://www.amazon.com/Kikkerland-Make-Your-Own-Music/dp/B000HAUEFY/