Some time back I gave a lot of thought to this idea. Ultimately I decided that a bumper carrier worked better for me. When I got a serious GF we started taking the TrailBlazer trailer on trips, so neither works at the moment.
Anyway, I got as far as a concept in SolidWorks. At the time I was planning on using some rubber torsions (because I have a loose set), but were I to build it now I might reconsider that - I don't know.
The design employs a bolt-on tongue that allows it to be flipped high or low depending on the height of the tow rig's trailer hitch. It also uses a 2" socket in the tongue for what ever flavor of hitch mechanism might be desired.
The goal of the bi-folding gate is that it function as a ramp w/o any other structure being needed, and w/o it sticking up rather tall at the rear of the trailer. I wanted to be able to see past it and as the plan also called for the trailer to be useful in a utility trailer manner no expanded or perforated sheet metal could be used. The plan called for a solid metal floor and sides, and that they be put in place in such a way that they added structure to the whole.
The gate's hinges are 1" OD heavy wall steel tube sections with a solid bar pin. Pin retention is done with a washer welded onto one end and secured by a cotter pin in the other.
After seeing what my grandfather did (in 1951) with GI sideboards I would likely look into doing the frame as a monocoque if I were to revisit this trailer concept, but since I now own this trailer there isn't much point. The sides and front of this trailer ARE it's frame. The 4' x 8' plywood floor sits on several light weight cross members and a perimeter lip, but is otherwise free to be removed. The astute may recognize the Ford Model A torque tube used for the tongue.