Ice box insulation
The standard "best" insulation is the extruded styrofoam, also known as Dow "Blue Board". Available in lumber yards in various thicknesses and not real expensive. Its a board-like material and can easily be cut or sawn. Also available in a pink variety. This is not the standard white styrofoam that cups are made of--same material, different process.
The pour in foam was commonly used in the past and has good insulating properties, but it should be sealed against moisture. It also is a tremendous mess. It gets on stuff and is very hard to clean up.