Here's an image of the underside of the trailer-hitch coupling using the universal joint.

The green thing on the left is the trailer, the green thing on the right is the bicycle hitch. I've shown the cold weld stuff, and an example of the bolt I used. The quick-release pins can be seen in the u-joint. The washers on the left side of the u-joint are there because here the bolt rotates relative to the trailer. The washers offer a surface for the bolt head and u-joint to rotate against. The bolt on the hitch end is held in place by the white plastic strip.

The second picture shows how the bolt has been made into a splined shaft.