Sand (much denser = sinks in water) has low thermal conductivity, and isn't much more expensive. Simply put, in two bodies of equal volume and 'heat capacity',

Sand (much denser = sinks in water) has low thermal conductivity, and isn't much more expensive. Simply put, in two bodies of equal volume and 'heat capacity', the one that's hotter stores more heat. Sand properties: [https://material-properties.org/sand-density-heat-capacity-thermal-conductivity/ https://material-properties.org/sand-density-heat-capacity-t...]