Building a bike trailer

I'm planning to make some bicycle trailers for various purposes. One of the best I've seen for simplicity and potential lightness is here:

With more component details here:

When I look at the way the transverse components are fitted to the spine, I wonder if it wouldn't be better using square section tube to reduce any rotational effects. This is without taking account of the difficulties which will be involved in bending the necessary curves in aluminium tube.

For bending purposes would packing the tubes with sand be the best method? How could the bending pressure be applied evenly and effectively in a D-I-Y situation?

Some aluminium tubes I've found on the Web are here:

-- Mycle

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