Not exactly what your looking for, but some of the tubular plastic containers for keeping matches waterproof will hold two 123 batteries nicely. I got one just for that purpose several years ago in the camping supplies section of Wal-Mart.
Don’t know if it’s necessary, but I drilled a small hole in it to vent off any gases.
And I guess you could cut it in half and tape over the end to store just one battery.
Alternately, if it’s just for emergencies, why not just wrap one battery up with heavy plastic film and duct tape. Cut it out if you need it.
Or some sort of durable coin container for dimes might also be easily modified for what you want.
FWIW, in my experience 123 batteries die pretty abruptly. Spares are a definitely a good idea. Spare bulbs are also a good idea (unless you’re using an LED).