This is a description of how to make a cargo trailer that attaches to the rear hub of a typical bicycle. The two main parts are the trailer itself and the hitch which is permanently attached to the bicycle. The relatively heavy "core" (near the axle) of the trailer was made from wood and common steel bracing and mechanical fasteners. Attached to the core is a system of aluminum pieces that comprise the main "basket" of the trailer. Most all the aluminum can be joined with woodscrews (to the core) and stainless bolts and nuts (for aluminum-only joints). I finished the wood with polyurethane and filled annoying gaps with epoxy paste filler. All the above parts can be found at a typical hardware store.
I also used certain recycled materials and added optional features using special new materials. I used a piece of tubular steel from an old ironing board (leg) as the angled connection between the bicycle hitch and the trailer core. I used a steel bar from an old exercise machine in the hitch, but that could be purchase easily. Also, I left space open in the lower portion of the core for a set of three batteries since I would connect my trailer to my electric bike. I see electric assist as a luxury, not a necessity. The wheels are 20" diameter (from recycled BMX bikes), but can really be any size you like, since the main body can be built to conform to any size wheel you have laying around. Two recycled backpack frames and some stretch-cords were used as a cargo lids, but these, like the batteries, are optional. I found a $10 grill cover that can be used as a stataionary cover as well as adapted to be used as a cover while using.
The aluminum was the most expensive part of this project and also added to the difficulty in assembly since it requires the drilling of many holes, some preferrably with a small drill press. Other that that, only typical hand tools are needed.