Saw your 2-inch by 4-inch boards in half.
Lay the four pieces flat on the ground in the shape of a square. Ensure that one parallel pair is tucked into the other (Reference 1). This is your floor's frame.
Nudge out the parallel pair that is tucked inside the other so that each piece sticks out a little from the frame (Reference 1). These edges will be the handles for your tap floor.
Cut your plywood square in half so that both halves measure 4 feet by 4 feet.
Set one of your plywood squares onto your frame.
Measure and mark your planks of bamboo flooring at every 4 feet (Reference 1).
Screw the plywood square onto your floor's frame (Reference 1). Use one screw for every 1 foot. Keep them 2 inches from the edge of the square.
Flip your square over.
Fill your square with the closed cell foam (Reference 1).
Screw the other piece of plywood over your floor's frame. Do not let your screws hit those you have inserted in the other side.
Put lines of floor glue on the side of the square to which you want to attach the bamboo flooring (Reference 1).
Place your planks of bamboo flooring onto the floor glue so that there is no space left between them.
Allow the glue to dry.