delete

You can usually find a way to disconnect the seat from the top crossbar to give you a little more space.

I was looking at one of these trailers last week. If you remove the blue vertical seat panel fabric which goes up to the crossbar, there is not much weight support left. The blue seat cloth is designed to support the weight of two small children, the black cloth panel under it does not seem sturdy with the blue part taken down. Also the top crossbar gives structure to the frame. If you pull one of these trailers apart you will see what I'm talking about.

I think I'd try lowering the blue fabric down to the front corners of the trailer so it can do its hammock-like thing. As it was meant to. Maybe a couple of big P clamps on the front corner bolts with strong nylon rope run between the two clamps, then run that rope through the "top" tunnel of the blue panel. Replace the top bar that now holds nothing just for frame structure and you should be set to hold a good amount. No extra parts needed other than a piece of rope and a couple clamps.