A structure for mounting a retrofit part to a cladding member may be used to prevent the retrofit part from causing a turn or positional displacement. For example, a clamp structure may include a clamp base and a clamp cover continuous to the clamp base by a hinge. The clamp may further include projections such as spike-shaped portions. The spike-shaped portions are formed and placed on both a mount surface of the clamp base and a mount surface of the clamp cover. The spike-shaped portions may prevent the clamp from causing a turn or positional displacement, and may be each protrusively formed substantially into a pyramid shape with a pointed end. The spike-shaped portions are formed to bite into an exterior surface of a cladding member and that resists external force, such as vibrations, when the clamp is mounted.