Protecting paper documents from fire is quite straightforward with, for example, a Sentry fire chest. This product has been tested hold the inside temperature under 177°C (350F) for 1 hour. This is fine for documents, since they will not combust or deform. However, I'm looking for an inexpensive solution for items that would take considerable damage at these temperatures.
Sure, I could house the fire chest inside another larger safe. Or build myself some kind of drywall, brick and mortar box to house the fire chest in... but I'm looking for something smaller.
My idea is, if I stored my valuables inside a stainless steel Thermos, could this offer sufficient additional insulation if put inside the fire chest? The plastics of the Thermos might deform at some point, but could I get enough insulation? A Thermos works both ways, but how well...?