Lovecraft

Yeah, you are right, my other option of sticking the water bottle in a thermos probably defeats the purpose too.

If it's a test to keep the water hottest for 5 minutes, could you use a meterial that heats the water bottle via chemical reaction like a hand warmer? That's what I'd do. I mean it's for 5 mins, not 24 hours. Is that cheating? It is a valid way of keeping the water hot, you are just putting energy into the system rather than trying to keep it from escaping in my scenario.