A calorimeter is a device used for calorimetry or estimating the heat of chemical or physical changes, as well as heat capacity.
Calorimeters are made of copper because copper is a good heat conductor, has a low specific heat capacity, and a light copper wall receives/delivers negligible heat.
Cu has a moderate specific heat capacity, so it immediately leads the equilibrium temperature by absorbing a small amount of heat. This small amount of heat absorption has no effect on the natural rate of the chemical reaction in the calorimeter.
Cu is very malleable, so a fragile box of small mass can be formed, reducing the box's heat capacity. This ensures that the calorimeter receives or emits a small amount of heat during heat transfer.