I think the steel gym locker would be ok. They usually have ventilation holes and are of a relatively thin gauge steel. If you leave the door unlatched or closed with a magnetic latch only, it would be even better. If the idea is to keep little hands out of the powder, then it may not be ideal since it would have to be locked.
The concept of the box is that it should not
be able to withstand much pressure before it opens; that's why a wooden box is desirable. If there is a fire, what does it matter if the box burns or not? The idea is that, if there is a fire, the powder storage box won't become a bomb.
SAAMI is the US authority on such matters. Here is what they have to say:
RECOMMENDATIONS FOR STORAGE OF
STORE IN A COOL, DRY PLACE. Be sure the storage area selected is free from any possible sources of excess heat and is isolated from open flame, furnaces, hot water heaters, etc. Do not store smokeless powder where it will be exposed to the sun's rays. Avoid storage in ares where mechanical or electrical equipment is in operation. restrict from the storage areas heat or sparks which may result from improper, defective or overloaded electrical circuits.
DO NOT STORE SMOKELESS POWDER IN THE SAME AREA WITH SOLVENTS, FLAMMABLE GASES OR HIGHLY COMBUSTIBLE MATERIALS.
STORE ONLY IN DEPARTMENT OF TRANSPORTATION APPROVED CONTAINERS. Do not transfer the powder from an approved container into one which is not approved.
DO NOT SMOKE IN AREAS WHERE POWDER IS STORED OR USED. Place appropriate "No Smoking" signs in these areas.
DO NOT SUBJECT THE STORAGE CABINETS TO CLOSE CONFINEMENT.
STORAGE CABINETS SHOULD BE CONSTRUCTED OF INSULATING MATERIALS WITH A WEAK WALL, SEAMS OR JOINTS TO PROVIDE AN EASY MEANS OF SELF-VENTING.
DO NOT KEEP OLD OR SALVAGED POWDERS. Check old powders for deterioration regularly. Destroy deteriorated powders immediately.
OBEY ALL REGULATIONS REGARDING QUANTITY AND METHODS OF STORING. Do not store all your powders in one place. If you can, maintain separate storage locations. Many small containers are safer than one or more large containers.
KEEP YOUR STORAGE AND USE AREA CLEAN. Clean up spilled powder promptly. Make sure the surrounding area is free of trash or other readily combustible materials.
SPORTING ARMS AND AMMUNITION MANUFACTURER'S INSTITUTE, INC.
P.O. BOX 218 - WALLINGFORD, CT 06492
Note that they do not specify the material for the storage cabinet only that it should be insulating and relatively weak.