What about taking some "U" bolts, and welding a piece of flat steel to them? Say 1/4" thick x 2"-3" tall and 6"-8" long. Weld them so when the u bolt is clamped to the pipe the flat pieces of steel are horizontal.

Then get some white UHMW, 1/2"-3/4" thick, 6"-8" wide and 6 feet long. Counterbore holes so they can be bolted to the flat stock that was welded to the u bolts. As the water level drops, keep adding u-bolts so the plastic can be bolted to them.

If you are worried about the plastic expanding and bowing in the heat, make countersunk slots and let it "float" on the attachment bolts.

The white corian countertops are a type of UHMW. If you know anyone that fabricates them, maybe you can get drops?

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).