My rack has what looks like a small license tag holder on the back that I use to attach my Cateye X3 rear light (TL-LD720-R). Like many rear lights, it uses an "O-ring" wrapped around something to secure the light. It works well on the rack, the seat post, or even on the trunk bag because there's a cloth loop in the back of the bag that I can thread the "O-ring" through. Even without the license tag holder, it could be mounted the same way on the rack frame because the Cateye comes with another mount for attaching to thinner/smaller diameter tubes or rack frames.