What you want, if it needs refractory-cement or whatever the insulation in the was of fire-safes is, is called a Forge or an Incinerator, maybe a Kiln or a Heat-soak Furnace, and no, an off-the-shelf Fire-safe is far from sufficient without abusrdist-levels of modification.

The heat needs somewhere to go, and those safes, the plastic home-and-office-grade ones anyways, are designed to take external pressure, not hold it in. In-fact, part of why they work is that the oxygen get's sucked-out by the fire outside -they generally aren't air-tight, save maybe the "flood-safe" ones ... which I still would be surprised to see protect anything versus prolonged submersion.

Even if you used an electric heating-element and seal the gap you need for that perfectly, you're going to out-gas the plastics and insulation, destroying the integrity of the safe and contaminating the area with toxic fumes, long before the contents are singed-enough to cause much damage without additional oxygen. Once you've got an open flame, your electric heating-element is fucked, so while you could maybe BS a fire-safe to suit your purpose, its one-time-use and closing/locking it is working against you every step of the way.

If it has to be cheap/free, and low-technical-difficulty, there's Primitivist channels on youtube that will show you how to make a forge from clay and/or clay-brick and mud.

I guess if heat-alone would do what you want, a dutch-oven over an open fire could do, or a charcoal-chimney-starter if the items are small-enough. For PID-document destruction, I use a fire-pit, or rather, I use bills and reciepts to light kindling, but I usually have enough of both that you could almost call the wood in the pit an after-thought.