Deformable interfaces provide unique interaction potential for force input, for example, when users physically push into a soft display surface. However, there remains limited understanding of which visual-tactile design elements signify the presence and stiffness of such deformable force-input components. In this paper, we explore how people correspond surface stiffness to colours, graphical shapes, and physical shapes. We conducted a cross-modal correspondence (CC) study, where 30 participants associated different surface stiffnesses with colours and shapes. Our findings evidence the CCs between stiffness levels for a subset of the 2D/3D shapes and colours used in the study. We distil our findings in three design recommendations: (1) lighter colours should be used to indicate soft surfaces, and darker colours should indicate stiff surfaces; (2) rounded shapes should be used to indicate soft surfaces, while less-curved shapes should be used to indicate stiffer surfaces, and; (3) longer 2D drop-shadows should be used to indicate softer surfaces, while shorter drop-shadows should be used to indicate stiffer surfaces.