Had two, one that bolted to rear axle, but it used to catch the rear wheel if you turned right to sharply, and another home built one fixed to seat pillar, witha piece of industrial hydraulic hose for articulation, both strong enough to carry an adult, one thing i learnt is pedesestrians dont look down, they seee you oass and walk out, some fell over trailer, luckily the times it happened i was stoped in traffic,
also a lower centre of gravity works beter for stability, eg try and make a box or tray below the level of the axles, and they do bounce around a bit, so if youre going down a bumpy track load it with something heavy like a tree trunk, then dispose of it other end when reloading.
Finally if using on a road remember the turning circle behind you, and space when passing vehicles, but theyre great and really strengthen your legs and arms due to the jerkiness of starting and stoping