When most people say direct heat, they think fires, but there is another way if you have a car, but it needs to be a metal container with a really good stopper, and you need space under your hood.

Using wire or clamps, SECURELY attach your bottle of ice to something under the hood of your vehicle. (I can't stress a secure mount too strongly- having a metal water bottle full of ice loose is BAD.) If it is running, or even recently off, that will thaw your bottle.

Or if your radiator is hot and the engine is off, just wrap the bottle in a rag, and wedge it between the radiator and the hood. Make sure you wipe it down with something like ethynol (NOT DRY GAS; bur prep pads from the FAK are ok) to get any gunk off of it.

I carry my water in my bag, so it comes in with me, but what my brother does in winter is partially fill and then without the cap on freeze his bottles before they go in his truck box. Capped, of course. He's a furnace tech so he keeps water in metal bottles year round- 2 minutes with the blow torch on low, and he's got water to make coffee. :P