I use a 20 litre container which I bought from a argricultural suppliers, but you should be able to find one in camping outlets and places like that. I fill the beginning of the day with water thats so hot I can barely stand it, and it manages to keep reasonably warm for the day (well it never gets really cold).
Also I use it for another purpose, as the cloths tend to get wetter more quickly in the winter, after each house, I'll drape the cloth over the container, and not only will the cloth be dry by the time I've finished the next house, it also acts a a insulator for the water! I use microfibre BTW, I only mention that coz I think they dry more quickly anyway.
Simon