During this summer's baseball season, I often took a reusable water bottle with me to the games, instead of buying (overpriced) water there (in a plastic bottle). The trouble was, the water would really heat up, sitting there in my cup holder, which was usually in the sun. So I made a little jacket for it.
Quite simple really. I measured the height and diameter of my bottle, and planned to make the jacket slightly bigger than those measurements. A cover fabric, a lining fabric, and a piece of Insulbrite to go in between for insulation. Then I quilted it a little bit to hold it together.
The next step was to sew a little fabric tab to the side, near the top, in case I want to attach it to a loop or ring of some kind. After this I sewed the side seam, pinned the "tube" to the bottom circle, and sewed the two together.
I sewed a casing into the top of my jacket, thinking I would put elastic in there to cinch it up. But that was not necessary. It fits so snugly that it's not going to come off unless I pull it off.
So I fill it with water and a few ice cubes and off to the game I go!