I had many sources for inspiration including one of the instructables' examples that uses a plastic bin, hose, spring and a quick connector. The first trailer I built went for minimal parts count using a piece of one half inch thick sheet of scrape plywood u-bolted to a seven sixteenths diameter axle and tires from a stroller. The hitch was made from a piece of bicycle tire wrapped around the rear basket handle and bolted onto the top and bottom of the end of a piece of conduit, that was u-bolted to the plywood. (see last picture below)
For the second trailer I also used a one half inch thick piece of plywood (three quarter inch next time) u-bolted to a one half inch axle and tires from a wood carrier cart, and a two inch caster wheel.
My original plan was to bolt the conduit to the metal frame running perpendicular to the axle (see first picture of last step) but I decided that I wanted only the trailer axle to carry 95% of the load. Put another way... I decided not to build a semi-trailer where a large portion of the load is carried by the rear tire of the bicycle. I used the half-inch plywood (new) to start with because it was handy, only needed a trim and was light.
The u-bolt arrangement permits the plywood to be leveled without bending the conduit permanently. The first piece I used had enough contact area together with a slight offset angle to keep it from slipping when the u-bolts were tight. I put a carriage bolt through the second piece of conduit (which had a straight end to fit through the PVC pipe) because it was too short to provide enough contact area with the plywood. Even though attached in a small area I expect the conduit will bend before the plywood will break, especially when I replace it with three quarter inch sheet. If the conduit bends or the plywood breaks from too much weight then I have not done my job in balancing or securing the load.