Cargo trikes can be constructed in several different ways, using rummage laying around.
Recently, I have found to be deluged with requests for information on the construction of the Cargo Trike: a "backward" tricycle with a cargo carrier on its front.
All I can say is that the people who build these economical light hauling rigs use whatever parts they can scrounge, and put them together however they can. The typical Cargo Trike shows little evidence of welding, with components wired and lashed together in an unsightly manner. (Clearly, there's no “right” or “wrong” way to construct one of these people-powered hauling vehicles!)
The single thing all Cargo Trikes have in common, it seems, is the way in which the trikes' frames are joined to their two-wheeled freight boxes. As a rule, each builder, cuts off the forward portion of an ordinary bicycle's horizontal framing tubes and welds a short length of pipe vertically to the front of the resulting stub frame. Then he inserts a 1/2-to-5/8 inch rod down through this pipe to serve as a steering hinge. (The rod, in turn, is held to the freight box via metal plates at top and bottom. Often, an extra brace tube is then added between the bottom of this “pivot pipe” and the original bike's crank housing.
Details of cargo box construction vary from one Cargo Trike to the next. Sometimes a pair of bicycle front ends (i.e., fork and wheel) are lashed or welded directly to the carrier's sides. On occasion, however, you'll see motorcycle—or even wheelbarrow—wheels mounted on a solid axle running beneath a machine's frame.
Perhaps I've raised more questions than I've answered...but I hope this info has been of some additional help to Cargo Trike fans!