Despite significant advances in ultrasonic instrumentation, accurate and repeatable measurements of skin structure and dimensions remain problematic. Skin structure is complicated and structural boundaries are non-planar and may be diffuse, inhibiting the application of inversion methods for skin thickness measurement. Coupling a transducer to the skin surface with water or aqueous gel can produce significant artefacts in thickness measurement, which may be prevented by using a rubber dry contact. This work describes a theoretical investigation of these problems, using the PZFlex finite element code as a virtual prototyping tool. Simulations show that non-planar and diffuse boundaries have significant effect on the output backscattered signals; and using of low loss rubber dry contact is possible but further work is required to determine the properties and optimum shape of the rubber interface.