Line your cooler with Reflectix (aluminized bubble wrap). You can find it at most home improvement stores. It was invented to insulate homes and buildings. Smart campers came up with the idea to use Reflectix to keep the heat out and the cold air in coolers. Cut the Reflectix into pieces that fit, lining the inside of your cooler, including the top/lid. You can even throw a sheet of Reflectix over the outside of your cooler to further insulate it.