I use a small 26 qt. roto molded cooler from Wally World.
Some things that have worked for me:
1. Use a main cooler and a day cooler. Cold drinks go in the day cooler. The main cooler stays closed, which preserves cold.
2. Pre-cool the cooler. Use block ice. Frozen food also counts as "ice" until it thaws. Fill air gaps with cubes.
3. Don't pour out the cold water from the melted ice. The cold water will keep things cooler than the air that replaces it. If you are concerned about food items being ruined from coming into contact with melted ice use ziplock bags to protect those items.
4. Reflectix or similar foil-backed bubble insulation wrapped around the cooler helps increases the R value. (Some ACE hardware stores sell it by the linear foot.) Anything that keeps the sun off the cooler and heat at bay will work, including an ordinary blanket, jacket, windshield sun shade, etc.
5. Only keep things in the cooler that actually need to be kept cold. In the same vein, beer for the day after tomorrow does not need to be chilled today, especially if you are in terrain where it can be pre-chilled in a cold stream before going into the cooler.
6. Did I say keep the cooler closed? It bears repeating.
On my last trip, mid-80's in the daytime and low 60's at night, with a 5 hour drive in 100 degree heat to get home, my cooler still had enough ice to keep everything cold at the conclusion of the 3rd day. On the prior trip, with cooler temperatures, my ice lasted 4 days. My record with my current cooler is ice into the 5th day, but there were only a few small pieces left - not even enough for a cocktail. Still, not bad for a $78 cooler.
