About three years ago, I spent about 150 bucks and three days in my basement. After all that, I ended up with a concrete gun vault with a steel plate door. All concrete is integrated with concrete floor and wall by first drilling into the wall/floor and hammering in 6" pieces of re-bar then building forms around them. The only thing I have yet to add is about a 4" thick concrete liner to the steel door.

Hopefully this will stand a fire. I'll let you know AFTER the ashes cool off.