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.
Step 2: Chassis underbelly
the dirty underbelly. this is a good detail shot of the chassis of the trailer. the chassis is made from 8020 extrusions of aluminum. this stuff is robust and re-useable, kind of adult lego, but it is expensive. you could similarly make the same chassis with pieces of timber, or even an old aluminum ladder might work well. nuts inside the extrusion are used to affix things. you can see the cross member plate is affixed in this manner, as are the axles at either side. The axles are 1/2in steel that fit to cheap wheels also bought from mcmaster. they were originally for a front loading tricycle I was building, but i got distracted...