Can you provide some info on the vertical bar on the rear of the rack that you're using for mounting?
Also, be aware that the MS taillight is prone to failure. The power wires as they are soldered into the circular PC board have no strain relief and are fairly small gauge. As you turn the ring, the inner portion of the light is not "supposed" to turn, but often it will move slightly because of the stiffness of the chrome ring. The internal power wires can, over time, break off at the PC board from repeated "tugging," and if you're unlucky like me, it'll be the positive wire, which can short out to the case and destroy the battery (depending on who's battery you're using). Anyway, just a heads up that if you like the particular mode that you have it in, you might be better off just connecting and disconnecting the battery to turn it on/off, or just be extremely careful when turning the ring that the inner portion (clear lens) is not also turning/moving.