Just did my this past spring too. Used treated 2x4's and outdoor carpet from Home Depot. Had them cut into strips sufficient to cover the top, two sides and a bit of over lap on the bottom (I wanted it open for drainage of the boards). I then used a staple gun with rust-proof staples and some carpet glue on the top and sides only to make the boards. After making them and allowing the glue to set up properly, I swapped them out for the old boards (boat tied up at the dock) using new galvanized lag screws. Still look like new as of this fall.