I had the need for a larger bike trailer, but little funds, and minimal tools in which to build one. I already owned a bike trailer that was once one of those "kid cart" type trailers that I had taken the cover off and reinforced with various pieces that once held the top cover/roof.
My project in mind was to take a welding cart and make it into another bike trailer, and that's when I decided that instead of making all new hitch parts for the welding cart, I would just combine the two carts into one for a longer trailer that could give me a bit more capacity, with the minimal amount of work necessary.
My main concern when building the trailer was that it would not be too flimsy as to warrant it unusable, yet easy enough to build that I would not have to seek out someone with welding equipment, either.
Step 1: Carts galore
The first bike trailer need to be taken apart so as to remove the top portion and make it as skeletal as possible for practical use. After doing that, I took one section of the metal bar that was once the top support and flattened the end of it so as to connect it to the cross beam in the front; I did this for added stability and to also have a bar that ran through the middle for internal structural support.
The original trailer conversion I also added wood to the top of it for a flatter platform.