Attach the 2-by-4 pieces to form a rectangle. The 8-foot sections should be on opposing sides, and the 4-foot sections on the other opposing sides. Use the decking screws to attach the pieces at the ends.
Attach the plywood sheet to the top of the rectangular frame using the same galvanized screws. Using a drill, drill 5/8-inch holes into the 8-foot 2-by-4-inch pieces. Drill one hole 1 foot from the end and drill another hole 2 feet from the first hole. Repeat for opposite end and again on both ends of the other 8-foot board. The rope will go through these holes and connect the floating barrels to the platform.
Tie a knot in one end of a 25-foot length of the rope. Thread the opposite end through the hole on one end of one of the 8-foot boards. Pull the rope through completely until the knot is tight against the board. Wrap the rope around the barrel multiple times until the barrel is securely fastened to the platform. Thread the end of the rope through the other hole and tie a knot in the end to secure the rope. Repeat for the remaining three barrels.
Apply a generous bead of construction adhesive to the plywood platform. Measure and cut a piece of marine carpet to fit the size of the platform. Apply the carpeting to the decking, spreading smoothly to ensure adequate adhesive coverage. The platform is now complete and ready for the water.