Hear in East Central Indiana several people I know including myself use big coils of that black greenhouse sprinkler tubing (it's ABS if memory serves) or polyethylene water.
You build a box in the neighborhood of 4' W X 4' L X 10" deep, leave box with 1 side still open, and paint the entire interior of the box black. Then coil in your black plastic retaining as necessary, place in an area of max sun exposure and plumb into outlet side of the filter loop.
I've seen boxes made out of all sorts of material, but pressure treated plywood seems to be most common, I've seen the plastic tubing sized from 5/16" to 1" used, the bigger polyethylene is easier to plumb in to loop in my opinion, some people will put the clear greenhouse panels on the open side to block debris, all sorts of variations of the same design that are pretty cheap, work pretty well and is super easy to add on too.
The best ones I saw were rolled sheet metal hoops and back plate about 4' in diameter with a metric shit ton of 3/16" holes. It was carefully layered with as much 1/2 tubing as would physically fit, with the outer layer was lashed to the metal hoop through the holes, then each successive layer was staggered and lashed to the previous layer, until the bending radius of the pipe got too small to hand form, then a center hoop was added to hold all the tubing snug in place, and the plumbing went out the center opening, the entire thing inside and out was painted black and had an acrylic cover. With 3 of those and he could keep an average size in-ground( plastic liner style) pool more than warm enough and would have to slow down the heating flow,(it was on a second pump and manifold for max temp control and he usually got 2 - 4 weeks more open pool time compared to everyone else that I knew had pools with no assist.
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