I'd be very wary about using any spacer on a new car because without an OK from the car manufacturer you may invalidate your warranty. The towbars have been tested and type approved with specific dimensions as laid out in the applicable standards and this includes the reach from the rearmost attachment point to the towball. By using a spacer, you will increase the bending moment in the towbar therefore also the loads in the underbody attachment points compared to the standard condition. If there is need to use a spacer for any reason, I would advise getting the vehicle manufacturer's approval in writing first.