This is my first instruct able, so please remember that I am only trying to share an idea that I found to do the task I needed.
I needed a light weight trailer that was strong enough to carry a variety of different loads, without a lot of maintenance. I also wanted to use all recycled materials and tried to keep the cost at a minimum.
I used these 4X4X1/4 aluminum c-channel wall supports to make the frame lightweight and strong. The dimensions can be whatever suits your purpose, my length is 40 inches long by 20 inches wide.
For the goose neck, I used an old bike frame that I had. I used a grinder to cut away every thing but the bottom rail and back wheel supports, then I spread the supports and welded a piece of flat iron on so I could bolt it to the frame, I used a 4 inch u-bolt to fasten the front to frame.
I used wheelchair wheels, because they are durable, cheap,( I paid 5 dollars for this set, less than one inner tube would cost), and they are reliable, no flats,
The blue wheel spacer is scrap 1x1 steel tubing.
I added 1/4 inch plywood deck .
Please remember to always wear safety goggles, and follow tool safety warnings when working with power tools