I think I'd be more prone to try the rice in a sock idea over a hot water bottle. The potential problem with a water bottle, is that after it cools down, it the water seems to absorb heat from you, which is the opposite of what you're going for. It may not be a problem though if the water bottle is wrapped in a towel or something similar. It's been a long time since I used one personally. On the other hand, once heated rice cools down, it doesn't have the same effect.