Having actually had a 50's vintage cargo trailer (in the 70's) that I used to deliver papers with I'll describe how it was done. The trailer had a full tube frame of mild steel. It attached at the seat post like a trail a bike with a very thick piece of leather sandwiched between plates so that it didn't pull out. The top rails were slanted to the back. with the hitch arm being supported but the top rail (think chariot). The floor was 1/4 inch think plywood and the rest of the trailer was covered in tent canvas.

I may have a picture I can post later. It had been recovered at that point but it will give you an idea of what it looked like.