Here's my current favorite cargo bike and bike trailer.
The wagon has wheelchair hubs with rims and pneumatic tires from a mountain bike.
The wagon frame is lashed together from hockey sticks and other sticks.
It's strong and easy to change.
It's survived years of heavy scavenging at [miters.mit.edu MITERS] and a couple of trips to Burning Man
The bike's front and rear racks are welded
on from pieces of bicycle frames and broken ikea chairs.
The rear rack has a simple cleat that makes it easy to lash the trailer on with innertube.
I hang two milk crates over the front rack.