The project is more complex than necessary, by the way. You could put all the notes in one track (or two if you run out of polyphony). I split this into many tracks so I could spread it spatially and so that I could sidechain its dissolution at the end of the piece.
At one point years ago I spent a lot of time with Shepard tones. They needn't just glide up and down; they can be treated like any pitched sound except that, if the sines are stacked at the octave, they don't have an octave characteristic. [i]Sounds created by stacking sine waves at another interval will repeat at that interval. So a sound created by stacking sines at the fifth or diminished fifth will repeat at the fifth or diminished fifth.[/i]
Shepard's original tones were made up of sine tones stacked at the octave and sent through a bandpass filter. The filter is unnecessary: if you start low enough and continue high enough, the limits of hearing will act as the bandpass filter.