Construction is easy if you use a corner clamp to hold the sides in place while you screw them together. Initially, assemble the box upside down. There should be 30 screws in all. Turn the box right side up. The platform is supported and stabilized by a combination of lifting and balancing floatation.
Balance is controlled by the sheets of foam inside the box.
You will find the that foam is too long to fit neatly inside the box. Make certain that the foam does not fill the box to the very top. The air gap helps prevent the plywood from rotting. Place the spacers on top of the foam. Ensure that the spacers will hold the foam firmly against the bottom of the box.