The human spine is a complex mechanism composed of both passive and active components. The nonlinear stiffness of the passive components provides mechanical stability to the system. There is a need for spinal implants that have nonlinear stiffness to provide this stabilization if the spine loses stiffness through injury, degeneration, or surgery. There is also a need for spinal implants to be customizable for individual needs. This paper proposes contact-aided inserts to be used with the FlexSuRe™ spinal implant to create a nonlinear stiffness. Moreover, different inserts can be used to create different behaviors. To show this effect an elliptical contact surface is considered and the inserts are varied by changing the semi-major axis of the elliptical section. An analytical model is introduced for insert design, and the model is verified by comparing the models force-deflection profiles to a finite element model and tests of physical prototypes. The models and experiments demonstrate that it is feasible to create a spinal implant that has a nonlinear stiffness, and that different inserts can be used with the base implant to customize the behavior for individual needs. The analytical model developed is a tool available for implant design.