I fabbed up a BLACK tarp and just lay it on top with four weights at the corners. When it's pool time just kick the weights off, tug it off by one corner and bunch it up out of the way. It isn't the prettiest setup but it is by far the lightest, easiest to handle and most effective:

Often it heats the top 6 inches of water so much that it's warmer than bath water. I have the suction set up to pull the hot water off the top,shoot it to the bottom and push the cold water up to get heated. Power of the sun with that black tarp is amazing.