I found some good info on storing of powder and primers a while back. Can't remember where I found it, but it was by a google search, so start there.
IIRC, it was recommended to use a wood box, but I don't remember the thickness. DO NOT STORE IN ANYTHING THAT WILL NOT RELEASE THE PRESSURE EASILY. A fire proof safe is even a no-no.
I have thought about building a box using sheetrock instead of wood (for the fireproof rating of sheetrock). Have panels that will pop out. The idea is to keep the fire out as long as possible, but to not keep it inside at all once it starts.
On one of my searches, there was a good report from a fire fighter that had responded to a gun store fire. Lots of ammo and reloading supplies. Ammo and powder all cooked off during the fire. Only one fireman was injured, when the ammo was going off, a primer flew out and cut his ear. Interesting read, made me not quite as concerned as I had been about the safety aspect, but it is still very important.
I started out with my reloading equipment in the house. It is now mostly in a detached workshop. I feel safer about that. I am also less concerned if, for some reason, my insurance company decides to deny a claim because of whatever loophole they can find.