1. Mark and cut the plywood pieces according to the cutting list. Cut the angled end pieces with a circular saw or tablesaw set to a 9 degree angle.
2. Cut the insulation the same size as the plywood base, then screw both to the 2 x 4 supports with 2 1/2" screws.
3. Screw the first layer of front and side pieces to the base and to each other, then add the back piece. Predrill the screws with a countersink bit.
4. Glue and screw the remaining front and side pieces on, using clamps to hold them together as you predrill and screw. Use 1 1/4" screws to laminate the pieces together and 2" screws to join the corners.
5. Glue and screw the hinged door pieces together, aligning the bottom and side edges, then set the door in position and screw on the hinges. Add a pull or knob at the center.
6. Paint the inside of the box with black high-temperature paint. Cover the back and the door with reflective foil glued with contact cement. Let the paint dry for several days so that all the solvents evaporate off.