A security guy I know told me that the best low budget gun storage for a DIY-type is to pull off the plaster or drywall on an interior closet and cover the studs and ceiling joists with a layer of chain link fence material, then shear panel with 3/4" plywood and plaster or drywall over the plywood. Add a fire-rated steel door in a steel jamb, with two deadbolts (differently keyed) or a deadbolt and a combo lock. Not perfect, but quite functional.