What got me back onto this project was GeekMomProjects' wearable battery holder:

https://www.geekmomprojects.com/flexible-3d-printed-battery-holder/

It takes about half the bulk of the pendant out and puts it on the necklace, which goes a long way towards meeting the stringent size requirements mentioned in the previous post.

(and now I can close that tab and come back to it from here)