Expanded polystyrene block.
Polystyrene is a great model making material as it is cheap, easy and quick to work with. It is used in architectural and railway modelling to create hills and other topographical features.
Use Foamcoat to provide a hard, durable finish that can be sanded & carved. It will also make the polystyrene flame retardant (not flame proof but prevents fires from starting or to slow the spread of fire), ideal for theatre and display props.