People always ask how it's connected to the bike...
Same thing as my brother's kiddie trailer - a stout spring, the ends held in steel tube. If you click into the article on metrompg.com, there's a zoom in pic that shows the hinge close up.
There are also some ball-joint type hinges you can make out of (apparently) common inexpensive hardware store parts. See this site for examples:
Community Bike Cart Design -
A simple, homemade, low-cost bike cart for car independence and community
EDIT: the cost of my trailer was an afternoon & about $23. Plus I got exactly the size & shape I wanted.