"Line your cooler with Reflectix (aluminized bubble wrap). 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.
Did you know, you can use common bubble wrap as insulation for windows? I had heard of this bubble wrap trick to save energy before, but had never actually gotten around to trying it. Not for every window, but just an idea to consider.