Measure the height and width of the opening for the door carefully. Mark two sheets of plywood with a pencil with the dimensions. Cut the sheets along the marks with a circular saw, and hold them together to verify that the sheets are identical in size and shape.
Hold the cut sheets of plywood in the opening to verify the measurements. Mark any adjustments needed onto the wood, and cut with a circular saw.
Cut two 1-inch-by-4-inch planks to the width of the door with a miter saw. Hold the planks against one of the cut sheets of plywood to verify the size. The ends of the plank should be flush with each edge of the plywood sheet.
Spread wood glue along one face of each plank. Press the glued face of the plank against one of the sheets so that one plank is flush with each side of the plywood and the top of the plywood, while the other plank is flush with the bottom and sides. Clamp the planks to the plywood.
Drive 1-inch wood screws through the plywood into the planks, spaced every 8 inches, with a drill and screwdriver drill bit. Remove the clamps.
Repeat the process with two additional planks to be positioned flush with each vertical edge of the plywood sheet. The ends of the vertical planks will sit on the inside edge of the horizontal planks flush with one edge of the plywood sheet. Repeat with one plank cut to the distance between the inside edges of the vertical planks and placed halfway up the door creating the figure-8 pattern. Attach the planks with glue and screws.
Press insulation, such as foam or Styrofoam insulation, into the areas between the edges of the planks to fill the space between planks. The insulation should be as deep as the depth of the planks. Spread glue over the exposed face of the wood planks.
Press the second sheet of plywood on to the assembled door. Clamp the plywood in place so that the edges of the plywood are flush with the edges of the assembled door. Drive 1-inch screws spaced every 8 inches into each edge plank of the door and through the center of the door into the center, horizontal plank.