Left luggage at Fez railway station
Fez station, in the new town, has a bag-minding service reached from outside the building rather than from the concourse. Travellers report a rate of around 10 MAD per bag per day for padlocked bags, open daily roughly 6am to 8pm. It is the handiest option if you are arriving or leaving by train.
What it doesn't cover
It closes in the early evening: a night flight or night train leaves you stuck if you have not collected your things by 8pm. Bags usually need a padlock you supply yourself, and both the rate and the hours come from traveller reports, not from an official ONCF page.

