A cookie tin? No. I think it will melt if a battery fails inside of it. But it depends on the battery type and failure. Most of what I know about this issue, I read from here or heard from some rc car enthusiasts. (Those guys have battery fires all of the time and are the primary reason why LiPo bags exist.) I know that Lithium Polymer
batteries, when they fail, will outgas at 1400 degrees +. I don't think a tin is going to survive that, in fact, I think it will make matters worse. That said, I'm not sure what temp the outgassing of LiMN battery is, but I'm going to guess it's pretty high.
An ammo box is what I'd recommend for a battery box. Or a LiPo bag, they're cheap and tested.