Not being a downer but I probably would stay away from cotton. It's a horrible choice for hiking. Once it gets wet it won't dry for awhile and loses all ability to hold heat.

You would be better off with a material that is at least water resistant.

This is what I use. It's pretty pricey but it sheds water pretty easily. Not only that but it's built more like a harness in case I have to lift him up for some reason(most likely in water). It being a vibrant color is another plus side.