Since no one has shared a solution, I will give it a shot.
The only place you have to attach a hitch is to the frame. A bad idea would be to attach it to the axle housing. Whatever the trailer did, the back end of the trike would react as well.
A frame mount must spread the load to as much area as possible to reduce adverse reactions caused by the trailer. The Sportster frame is not very strong at the rear where the fender struts are located. The fender struts themselves would be insufficient to support the load multiplied by the hitch so attaching where the shocks are would really help.
Lastly, a Sportster is the last pick I would make to pull a trailer on a wet road .....:cxtv: