One of the quickest ways to make an ammo box in Fortnite creative is to copy one from a prebuilt structure.
One that we’ve found was the village clock. Place the village clock on your creative map and then copy the ammo box in the bottom. Once you’ve copied it you can then delete the village clock unless you plan on using it.
You’ve now got an ammo box that you can copy and paste as many times as you want throughout the map.
If you’re not to bothered if it looks like an ammo box or not you can just set a chest where there is only ammo in it. This way you get to set whichever ammo you want instead of getting a random piece of ammo from the ammo boxes. But this all depends on your intended use for the ammo box.