Measure to the center of one of the panels and cut out a 6-inch by 6-inch square from the middle using the jigsaw. This will be the center panel to which the speaker is mounted.
Measure to the center of the remaining four panels and draw a horizontal line across the middle. Pipe a line of glue along this line and stand the panel with the cutout on top of it. Repeat this process on all four sides of the cutout panel until you have a space made of 2-foot walls, with the cutout panel forming a "floor" 1-foot from the top and 1-foot from the bottom. Allow the glue to dry, then reinforce the assembly with nails.
Cut the circular panel of plywood into quarters and glue each quarter around the 6-inch opening in the cutout panel. The right-angled corner of the quarter-piece should sit in the joint between the cutout panel and the wall of the box, one foot from either side. This will form a curved ridge that runs from the mouth of the box to the mouth of the smaller cutout.
Use the saw to make a series of parallel scores on one side of the plywood trapezoids. Be careful not to cut all the way through the wood, the idea is to score it to make it flexible. The scores should be around 1/4-inch apart, running horizontally from the base to the narrow end.
Glue and nail the wide base edge of the trapezoid to the outer rim of the box. Use the curved circular pieces to guide the shape of the flexible piece down toward the cutout in the center panel. Secure the narrow edge to the rim of the cutout. Repeat on all four sides so you make a sloping "pit" down to the cutout from the front of the box.
Flip the box over and attach the speaker to the rear of the cutout panel using the provided mounting screws, positioning it so the cone is right in front of the cutout.