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This page is for a general answer and it consists of at least two main parts: personal experience and technical part. The technical parts are perhaps the two hardest parts.
Has anyone attempted to make a cargo trailer for their bike? How did you do it? How has it worked? How long did it take? Would you do it again?
Technical parts with their own separate questions
I've made several and documented at least some of them:
I much favour the mass produced design because it's easy to weld or braze together and quite robust. The chain-in-spring hitch is reliable and durable and very hard to break (the chain links eventually wear though but that takes years).
I made a trailer similar to this one. It's based on using a second hand aluminum backpack frame and lightweight conduit. This one connected very simply to a carabiner attached to the seat post. At the time I had access to a drill press which simplified things for someone building it alone - you will want help keeping things aligned if you don't.
A friend built one out of a large container like this very simply - again connecting to the seat post.
Haulin' Colin has built about 30 trailers using a fairly simple MIG-welded steel box-frame design using square tubes. I can attest to hauling several hundred pounds in one easily, with uphills being slow and downhills pushing the limits of my bicycle's braking system to the limits.
Recently Duncan Cycles has been documenting his construction of a TIG-welded steel-tube version of the same design.
If you have the know-how and the equipment, welded or brazed designs are going to have significantly higher strength for the same or lower weight as a bolt-together design.
I also helped with this monstrosity of a trailer built around a shopping cart specifically to win the Idiotarod. I wouldn't recommend it unless you absolutely need to incorporate a shopping cart into your design.
The guy's at Umwelt und Projektwerkstatt Freiburg e. V. (Germany) are developing a three wheel trailer for heavy loads. It's named Carla Cargo and it's Open Source Hardware.
Their documentation is just at the beginning (they spend their time more with building and trying out new stuff, than with documenting) and until now only in German.
The dimensions can be seen at Werkstatt Lastenrad.
I paired a Trail-Gator seat post hitch and a quick disconnect tie rod end like this one (http://www.midwestcontrol.com/series.php?id=16) that lets the bike lean while keeping the trailer flat on the ground. If memory serves me correctly, the stud thread is 5/16" (8mm). I'm also sure that McMaster-Carr carries the same type of connection.
The trailer I built was also inspired by a Burley Travoy, so towing children is not an issue in this case.
