My bottle had some very good insulators. I used foam, paper towels, bubble wrap, and fleece. I know they were good insulators because the hot water in my bottle started at 37.6 degrees Celsius to 38.7 degrees Celsius. If my temperature went up then they must be very good!
I don't think there is anything I could of done to make my setup work better. I know my setup went well because my temperature not only stayed the same but it went up!