Save Time Online
For a wacky prompt to make hold music for cars outside of a tunnel, I did the whole thing with the safe chord progression, mid-slow tempo, and announcements about how you should hang up (in this case leave the car line).
There's a tension between making something really sound like hold music and making something you don't want to hang up on. In order to avoid hard decisions, I went with always making it more hold-music-like, except for the vocoder.