Fireproofing questions

Hi all, I just scored a 6-foot metal, 2-door cabinet that is marked as fireproof. I'd like to think it'd be a great place to store classic media, but it's not entirely heat-proof. To test it, I placed a hot clothes iron on the outer wall and measured the temperature/rate of climb on the inner wall, as well as the ambient temperature inside the cabinet. It was enough that, even with a comparatively small heat source (well, to a garage fire, anyway), anything within a few inches of the inner wall would have most definitely been damaged, and possibly items close to the center after a half-hour of exposure to the heat source. My question is this: is there something anyone could recommend that would make a good lining for the inner walls of the cabinet to insulate it from heat? Something that doesn't take up too much room (would kind of defeat the purpose of having a cabinet to store things in) and would protect the interior contents in case of a small fire. I don't think there's anything between the inner and outer walls besides air, though I'm not certain. Perhaps I could squirt some of that expanding foam in there or something? Cheers, Aaron