Rear racks should come with an easy way to mount a tail light. Over the years I've had to make my share of custom mounts, which were usually made from oddly bent pieces of aluminum, which were designed to work with a specific light. If I wanted to change tail lights, then I had to make a new mount. Tired of having to deal with changing mounts, I decided to make a generic one for the TerraTrike rack. It's quite simple and most any tail light designed to clamp to a seat post will work with it.
The round black thing is a length of Delrin that I cut and center drilled, but a short piece of tubing can be used in its place. I just installed the pictured mount today, and have been using one made with an old brass bushing for the last few months. If I didn't already have some Delrin laying around I would have just cut a short length of aluminum tubing.
Since Delrin is a bit slick, I used a star lock washer where the Delrin meets the bottom of the rack to keep it from turning. Everything else should be obvious in the pictures.
TerraTrike or some third party should make this a simple bolt on accessory kit for their rack.