The client is very active, physically, and has expressed concern
that the necklace might catch on something...
You are asking for alternatives: Your customer is aware that s/he is
not compatible with a necklace. You might want to consider refusing
this job. If anything, really anything, happens to the necklace
within the next century or so, your customer will come back to you
requesting free repair by saying "I have told you...".
Maybe a dogs collar...