DIY trailer for a bike
Google "pedicab trailer hitch" and you will get thousands of results. I discovered a variety of homemade designs. The one I decided upon is pictured here. The key component of this design is the "spherical" joint. The ball joint is essential for providing movement on X, Y, and Z axes. Trust me on this, the hitch needs to allow the trailer to move left and right (X axis); up and down (Y axis); and to tilt when the tow bike leans in to a turn (Z axis).