The invention relates to a base comprising a closed cell foam core sheet having a closed cell foam layer with a density gradient between the center of the core and the surfaces of the core. The invention also relates to a method of making such a base by adding a first concentration of a first blowing agent to a first polymer to form a first mixture, adding a second concentration of a second blowing agent to a second polymer to form a second mixture, and extruding the mixtures to form the closed cell foam core sheet. The invention also relates to a method of making such a closed cell foam core sheet base comprising the steps of adding a blowing agent to a polymer to form a mixture, melting the mixture, extruding the mixture from a die, and rapidly quenching the mixture against a high heat transfer surface.