My daughter got bored with riding on the back of my Xtracycle with my son and wanted me to find a way to attach her tag-a-long to the back of it. Luckily, I have a pretty talented neighbor who designed, and fabricated, a bracket "system" for me.
The rear post attaches in three places to the uploader bars. It allows quite bit of vertical adjustment for the tag-a-long to attach to. My neighbor was quite concerned about "low amplitude fatigue stress" that the system would put on the bars because they are made out of aluminum instead of steel- unlike the rest of the Xtracycle kit. As a result, he built me an extra brace for the front of the rack to tie into the frame.
The end result is the sturdiest thing I've seen yet to attach her bike to mine.
I don't know how to add photos to a post like this, but I did upload some to my photo album on here for some more close-up shots of the rig.