A box constructed of drywall can buy you some time. Wallboard (?) of the type used by big hotel restaurants work even better (they're designed for stoves/ovens which are going 24 hours a day). The more the layers, the greater the insulation. In the '80s, some safe manufacturers use to use ceramic tile to insulate their safes.
Figure out the response time of your fire department, double it and build your box to that standard.
Vigilantibus et non dormientibus jura subveniunt