After several variations and the absolute requirement to have ice cold water available all day for hot Texas rides, I stuff a small 'pocket' ice box in the tank bag. It rests on top of my rain suit top and the bag expands enough to have some top space for the usual devices, gloves etc.
I fill the camelback bladder with just a little ice and top it off and chuck ice in the container. I have ridden 10 hours from Dallas to Lincoln in August and still had ice cold sips at arrival. The hose fits well between the two zippers and can be stuffed back into the cold environment with the left hand so there is no hot flush of heated water in the tube before the cold.
It depends on where you place your value. Tank bags serve well for many things but I ride with no electronics and Texas can bake you like a brownie before noon some days. Water is paramount in my book.