Update 4/23/08: I have removed the (removable) backpack frames from the roof and installed a permanent, relatively waterproof, "hardshell" lid. I've added two pictures to the instructable - the large white panels of plastic are a giveaway as to which are updated pics. The lid is lined on the top with garden/hardware mesh (plastic coated steel mesh, not visible in this picture) for placing things on top of the lid. Under the metal mesh is a thin layer of white backsplash plastic material (visible in this picture)- this same material was used to enclose all the sides as well. The hinge is a piano-hinge made of brass. I used a (home) window lock as a latch to keep the lid down and eliminate vibration of th lid at the same time. I used L-stock and flat aluminum once again as structural support across the perimeter and inner field of the lid itself. I found that quality (high in acrylic) construction adhesive ($5 per big tube) works great as an all around waterproof and strong glue for almost any of the materials used - it even sticks pretty well to the plastic - (but don't rely on a plastic bond.) The white plastic sheeting is about 1/16" thick and should be able to be purchased as 4'x8' sheets (<$20?)from most home centers, though I only found it at Lowes. It was best cut with beefy scissors. In general, I attached the plastic it by using 1/16"x1/2"-wide aluminum strips on all edges - glueing them with construction adhesive while using little stainless steel bolts every 16" or so. Sometimes I sandwiched the plastic (edges) between aluminum L-stock and flatstock - always using construction adhesive to seal all joints - to keep vibration down and seal water out. Note that the plastic is neither really light or heavy - it seemed well suited for my beefy trailer in terms of weight. As with all aspects of my trailer, this took a long time - probably 20 hours - a lot of it trial and error - testing and scrounging for parts. I guess the main thing I want to convey is that the combination of aluminum stock, steel hardware mesh (gardening section of store) and plastic sheeting can be made into a nice waterproof lid for a cargo trailer, provided you have the patience of dealing with attaching it with little bolts and construction adhesive. Once more thing - I used thick, tough, tubular (with a flat section for attaching) weather stripping as a sort of anti-vibration / water seal / drip-edge on the underside lid perimeter (visible in pic).
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.