Like many others I have turned a walk in closet into my armory/reloading room.
I have a regular door lock on it. Since the door open outwards that makes it slightly more difficult to break into it. But as you already said it only keeps honest people honest.
I'm really fortunate because it is the only closet that also happens to have a ventilation duct for heating and cooling. That helps with the powder storage.
Experience is a very bad teacher because it first gives you the test and then teaches you the lesson.