this is a good detail of how the trailer attaches at the seatpost.
this is the finished bicycle trailer. I built it principally because i had all the components lying around and needed a bicycle trailer. many substitutions for the different parts are available, so this is more of an overview and general conceptual how-to on building bicycle trailers.
Step 1: Plastic bin and crossbeam
I had this large plastic bin that comes with it's own top so the trailer could be enclosed and waterproof. The wooden beam accross the top was because the sides of the plastic weren't strong enough to really hold their shape. conveniently I was able to put rope cleats tapped through the side of the box into this wooden beam so I also end up with a convenient point for tying things on top of the lid of the trailer as well as securing the lid.