The technology relates to lens assemblies for sensor units that provide a low but consistent preload force over the entire operational temperature range of the device. Consistent preloading helps to avoid cracking and plastic deformation. In particular, a compliant structure of a polymeric material is able to expand and contract across temperature extremes. In addition, the polymeric material is arranged in conjunction with a retainer ring to form a discontinuous seal with the lens. This provides in a leak path that is able to reduce condensation or contaminants. As a result, moisture within the sensor unit is permitted to escape, reducing or eliminating impairments on the lens or other parts of the sensor unit that could otherwise impair device operation.