Having no visible seat post to mount a second rear light caused some head scratching. The rack has a light mounted on it, but I wanted another.
Here’s what I came up with
A spare rack part, a bolt, an old Burley connector (plastic pipe and a drill would be an alternative) and a standard light bracket.
The rack part (struggling to think what they are called) is bolted with two bolts to the rack, preventing it from moving in any direction. The Burley connector is bolted to the rack part which also has the light bracket holding it from moving.
It vibrates a little, so a more stiff joining piece of metal would be better (the rack part). The Burley connector could be cut down an inch too.
But it works.