Nice work all around!
Just a comment about the hitch however: the reason why many hitches are some type of rubber elastomer is so that they can bend and twist horizontally and vertically just in case you ever crash - you fall but the trailer stays upright. The other problem is if your cart ever tips, it seems your hitch, in its current iteration, would probably pull you down as well. But I can't imagine you're going that fast anyway.
I get your desire to recycle as many parts as you can though. Maybe you can rig a hitch with old tire rubber?