I've been freezing bottles of water all summer (it's been too hot!) with no problems. All you have to remember is not to fill them to the top. I've got quite good at filling Evian bottles about 2/3 - 3/4s and freezing them solid - no leaks, no splits, no problem. The plastic has a bit of give in it. Then they thaw with ambient heat during the day. I'd think engine heat, or heat from a fire at a distance would be OK too.