I actually made one of these for a pop up pool I put in our backyard when the kids were younger. It was a good size pool that I stuck behind our detached garage but it was in the shade pretty much all day. I had a small recirc pump pumping water during the day and it honestly was amazing how well it worked.
A DIY Solar Water Heater - Part 2, by St. Funogas. Once you’ve completed the coil, weigh it down with blocks so it stays together as you add a wooden “X”.