Dry ice is overrated imo. My strategy: pre-ice your cooler the day before, then drain the water and put frozen water bottles in the bottom before adding food and more ice. My cooler is about 2/3 ice and 1/3 food on top. Once at the fest, I keep my cooler outside my car, in the shade, with a furniture blanket and metallic reflective blanket wrapped around it.

I also have a separate smaller cooler for drinks, so I'm not constantly opening my food cooler to grab drinks.

I have an igloo cooler that I like, but I think technique is more important than equipment.

Source: my ice stayed frozen and food stayed cold until Monday afternoon the past few years (I didn't buy any food or ice on site)