I have two other tips that I didn't see mentioned.
I also freeze the large (1 litre size) tetra pak juice boxes. They stay frozen a long time and make great ice blocks, then when you're ready, ice cold juice.
On very hot trips I wrap the exterior of my cooler with one of those soft, flexible windshield heat reflectors, helps to keep the sun off the cooler too, especially if we're day tripping and taking the cooler in the tug with us.
Gerry, your rainbow trout sound yummy! Glad you had a great trip and some luck fishing