It's pretty easy, just take a saw and make a million cuts, and it will roll halfway decently. We use a radial arm saw. Here's a random video showing the concept.
Excellart offers a service, I'd probably recommend ordering a kit from them to get an idea of cost and how a big company would do it.
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All that to say, for something like this, I'd make 2 channel letter circles, bolt them to either side of a 3" square tube, then skin both pieces together with one piece of .040". This is the cheapest/fastest/lightest way to make a ~4' circle, anything bigger and you need a more substantial frame.