I wanted to have a look at the varoius ways in which a trailer can be connected to a bike or trike. methods at the bike end of things include couplings to the left chain stay (or both), the rear axle bolt (or both), the seat stay or the rear luggage rack. Here's what I've found. As ever, novel solutions from your own projects are always welcome by comment or email.. Also instructables.com is a great source of pictures and videos to give you help and inspiration for your home-built.
This blue trailer found at Zedomax is made of wood. It clips onto a custom wooden box on the back rack. Seems a bit lightweight to me though. An advance on this idea is to use horticultural grade hosepipe between the bike and trailer as a universal joint then put a brass quick release hose coupling in the middle. Simples!
This idea is illustrated here complete with .PDF plans to download.
Let me know about your homebuilts soon.