Another safe has been added to dungeon.  We never got around to putting the safes on a concrete pad and now with 3, it would be close to a yard of concrete for how I would want it.  And I really don't want anything permanent.
So I thought of putting a couple 6x6 or 6x8" under the length of all three and lag bolting the safes to timbers.  
My next thought was of a fire burning them and the safe tipping over, but drywall all around the base should keep them fireproof, right?

By the time a 6x8 timber on the concrete floor of the dungeon underneath large safes is burning, your safes and their contents will long be toast anyway...