I used a single layer of plane ol' 3/4" thick sheet Styrofoam with Styrofoam adhesive in a calk type tube (cost ~$3.50 a tube).
At about 1:30 today the adhesive was hard enough so I put a couple of frozen water bottles in the cooler. Put the lid on it and set a piece of wood on top for weight. (No better seal than that.) I put this in the back of the truck in the shade but outside in the heat (94° today). I also put another bottle of frozen water in the bed of the truck next to the cooler as a baseline. At 6:30 this evening the water bottle left in the bed of the truck was lukewarm, no ice and no coolness left at all. The 2 bottles that I put in the cooler will still mostly frozen with very little liquid as you can see. Obviously it worked. Of course I would want to coat the foam with epoxy or something and if I was making a serious cooler for fishing and I would probably use 2 layers of Styrofoam.
