This is not a traditional welded safe but Jerry outlines some of the advantages of going this way.
Maybe I'm paranoid but I like the idea of installing this yourself so there's no need for strangers to be in your house.
https://www.snapsafe.com/modular-safes/
SPECIAL FEATURES
SnapSafe modular safes are easy to assemble and move anywhere. Simply unpack them piece by piece and assemble them close to your desired location. The SnapSafe modular assembly system locks the walls together from the inside, making them as solid and impenetrable as conventional, welded safes. Their 2300°F fire protection offers peace of mind knowing firearms, jewelry, currency and valuable documents are protected from fire and theft.