I use my rack flexibly, sometimes with stuff lashed to the top, and sometimes with panniers. Either way, mounting a light is a challenge. Seat post, seat stay - either way there is some chance that I would interfere with the projection. I was able to mount a Mars 4 to the back of the rack pretty well; however I just got a B&M Toplight that I, with some trouble, mounted there instead. I now have the Mars mounted on my helmet. Last week I mounted a blinky on the axle nut of my IGH. It worked pretty well and did not interfere with panniers; however you would need a screw driver to remove in order to take the wheel off. I am now thinking about fabricating a custom mount.