Having perused a number of websites, I opted for agricultural tubing coils as very low-tech with materials easily and cheaply available in Oman, manufactured locally. I also decided to put all the coils out of the way on the flat, concrete roofs of a couple of out-buildings, rather than on the ground where kids could damage them. So here is the plumber trying to configure the initial trial coil. Turned out to be quite tricky and manpower consumptive in order to avoid kinks in the tubing. The arms above are mild steel, with considerable flexibilty to allow the agri-tubing (15mm diameter) to be slipped under. Each arm has four 2cm high, square-section, 7cm long pieces welded under it, to act as spacers. The two arms therefore end up defining a coil that has an inner diameter of about 54cm and an outer diameter of 226cm. We screwed in the center to the roof and then slipped in the agri-tubing under the long arms. Turned out that each agri-tubing coil as bought from the store is 250m long and fits nicely under the 250cm diameter arms. So we decided on each solar collector coil to use up a factory-stock 250m coil, collection area per coil comes out at 3.75sq m. Hopefully, with all coils having the same length tubing, we will have the pool water going through all coils, rather than bypassing some that may have longer coils and short-circuiting via shorter coils.