Make:

Here's DIY bamboo bicycle trailer you can email for the free plans... -

Materials
You'll need eight lengths of material capable of withstanding bending and compression, tubing or angle iron are ideal. You also need eight "spokes" that are good at taking tension. These could be wire, chain, or steel strip. Exoticists could use Titanium tubes and Kevlar rope. Two wheels from a donor bike are also needed.

Basic Layout
Seven tubes are pinned or bound together where they cross to create the basic frame shape that looks like a 6. This structure can distort like a parallelogram, or warp like a sheet of paper.

The Revolutionary Bit
With the addition of the eight spokes and the last tube this floppy structure transforms into a stiff light structure reminiscent of a square bicycle wheel. Imagine the square load area as the rim of a wheel, with 8 spokes connecting from its corners to a central hub. Truing these controls how square and flat the trailer frame is. The arrangement can be dished to make it flatter on the load side than the underside. Four spokes stops the structure from parallelograming while the addition of another four and the hub stops the frame warping. The top set of spokes could be replaced by a plywood/metal sheet, that forms a load bed.

Related:

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Conduit bike trailer - Link.

Homemade bike trailer - Link.