Timestretch MusicBox

Now we like to use a music box cue that has to slow down as the tension of the box powers out.
I found this little hack at YT to combine the pitch shift and master-pitch combination, which works OK but not in a very good quality. I wonder if I can just produce different tracks in hi-Q and then just fade into those - thus creating a slow-down fx.

Or is there a better way to achieve this?