I have seen systems where they coiled copper tubing in a big spiral in front of a room fan as a radiator. It probably won't work as well as a tube and fin or stacked plate radiator, but it will work.
As a tip, to bend copper tubing, fill it with sand first. This will prevent it from kinking and allow you to get nice curves. Of course, you have to get the sand out once you are done, but nothing is perfect.
El
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