[dbaupp]

Using a solid thermal mass is discussed on the linked wiki: https://fridge0.branchable.com/thermal_mass/

composition

Water is the easy choice.

A material that can be frozen would be better, because it takes a lot of extra heat to melt a frozen material and so more cold could be stored.

However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall.

This is an open research area.