I use a cooling sleeve called a 'Frio.'
It's an evaporative cooler. There's an inner sleeve filled with crystals that you soak in water every day or two. The crystals absorb the water and become a gel. As the water evaporates through the outer sleeve the insulin is kept cool. The trick is where you pack the sleeve. Just inside the lid of a black waterproof tank bag is a bad idea as I discovered unpleasantly. Number one, it limits the evaporation. Number two, it makes for a high ambient temperature that's more than the it can handle. Somewhere closer to the air is better.
There are also some small 12V cooling devices available (google "insulin cooler"), but not much as small or inexpensive as the Frio. I can't remember where I bought mine (maybe amazon), but it's a British product with only a few US distributors. Reviews were controversial, but I've been happy with mine. You just have to be smarter than it is.