I have a strap at the back of my seatbag as well, but if I put the light there it would sag downwards, directing the red light toward the top of my rack, rather than towards the eyesballs of cagers.

For now I am using another light I had around in the garage; the seatpost attachment is more of a plastic strap than a clamp, so I have tightened it around the back of the rack.