Ask your lumberyard to cut a 4-by-4 into a pair of 3 1/2-inch cubes and a pair of 2 1/2-inch cubes. Smooth edges and corners of cubes with sandpaper or a file (so they won't snag rope). Paint cubes same shade as rope, and let dry.
Each large cube needs 40 feet of 3/8-inch-thick twisted nylon rope or 35 feet of 1/2-inch-thick rope. Each small cube needs 35 feet of 1/4- inch-thick rope or 30 feet of 3/8-inch-thick rope.