Differential temperature alarm for fire-safe

I have a large (40AH) Lipo battery which I keep in a fire-safe, but the fire-safe is no use to me if all it does is delay any conflagration by silently containing the fire until the fire-safe itself is compromised, which with a large Lipo it undoubtedly will be. What I need - and what I can't find - is a temperature alarm that sounds when the outside of the fire safe starts to warm up, i.e. while it is still containing the fire within. I see a need for a product with two temperature sensors, which alarms when one of them is a preset number of degrees warmer than the other. (The control sensor being placed somewhere similar to the fire-safe location that would be expected to be at the same ambient temperature as the fire safe.) The difference need not be very much - maybe 3 or 4 degrees would be enough to avoid false positives. The temperature sensors would be on relatively long wires (eg 6 to 10 feet), so that the device was not located immediately next to the fire-safe in a way such that the device itself would also be compromised in the early stages of a fire.