Dan
I like the idea though. I was also considering using 55gal drums cut in half. Or thirds. Or vertically.
Since the soil is like 99% sand, soil integrity is my biggest concern.
I think chinampas worked because they were layered and the dredged soil was mostly silt and clay. So combined with layers of organic matter they formed pretty dense soil.
Our soil is beach sand with some organic matter in the form of silt/pond muck and humus. So the sand is pretty coarse and when wet melts.
We could brace with wood or some thing ridged but large scale application is out of the question. The area is too large and it is difficult to organize labor parties.
The other option is to wait for the area to dry out and build the raised hills and plant vetiver grass around the base of the hills. But we can not wait for the area to dry out