Ideally the shade cloth would be supported a few cms off the polycarbonate roof to reduce scratching, heat stress and provide an extra air gap for insulation. A good design would also be able to be automated in the future.
My current design uses the standard roll up blinds trick of a loop along the outside curve, around the roll and back up to the gable. This will stop the shade cloth from rolling down, but it doesn't stop things flailing about in the wind. So we add another string that winds up inside the roll and drops down to the ground.
We could support the shade cloth on a row of tensioned wires, though I probably won't bother.
A nice property of this design is that the total string required is almost constant, so we can simply return the string back from the down string to the up string. We need to add a bit of tension to the string to stop flapping. I prefer gravity tensioning as it is less prone to oscilations, so I propose adding a few kg to a roller hanging off the return line. To automate this design we would just need a motor driving the return line.
