Hugh:
I did some consulting for a well-known gun safe manufacturer. They make a premium product, and their clients never suffered loss due to either break-in or fire damage. The only heat resistant insulation they used was common sheetrock. They used multiple layers with offset corners and edges, similar to the wall between living space and garage in a modern home. They did not tape or use any mud, just a snug fit. It seems reasonable that the heat resistance of an ordinary inexpensive safe could be enhanced by fabricating a welded enclosure insulated with sheetrock and inserting the safe inside this outer shell. Remember: most safes are not water resistant! If you have a raging house fire, your valuables may not get burned up, but they may be water damaged during firefighting. – +P+