Thermoclear 1/4 in. x 96 in. x 1/4 in. (6mm) Thermoclear Polycarbonate Multi-Wall U-Channel PCTWU-6MM - The Home Depot
Outwater Plastics Black High Gloss 5/32"Poster Frame Plastic U-Channel/C- Channel 46 Inch Lengths (Pack of 3) - Walmart.com
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Outwater Outwater Plastics 341-cl Clear 3/4'' Rigid Vinyl Clear Plastic U- channel/c-channel 36-in Lengths (pack Of 4) in the Furniture Moulding department at Lowes.com