How to mount ZWave switch on stone wall - either retrofit in metal box or better?
Thanks for the suggestions in the comments.  I tried various lazy solutions, in the end I had to do it right.  Nothing else worked.
I moved the existing metal box close to the ceiling, I had to keep it involved because one of the wires coming into it is armored.  I then ran NM cable from there to the ZWave switch location and bought a surface mount PVC handy box for it.  I also moved it a few inches away from its original location which was directly adjacent to another metal box.
That all this stuff is mounted on a stone wall did not make it a fun job, which was part of the reason I tried to take short cuts but they just didn't work.