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Cut four panels from the foam board insulation. Measure out the pieces you want, starting with a face board that 10 inches wide and 6 inches longer than your window (3 inches on each side) so it will clear any curtain rods or molding. Draw your measurements with a straight edge first, score the foam with a utility or other sharp knife and then break the pieces along the scored lines.
To form the foam box, you'll also need two side pieces the same width as the face (10 inches) and 5 inches long. Cut a top piece that is 5 inches from top to bottom, width-wise it should be just a little shorter than the face to allow room to attach the side pieces.
Run a bead of hot glue along one of the long sides of the top piece. Hold it in place for two minutes so that it forms a right angle with the face piece (Image 1). Glue on the two side pieces to form a rectangular box with two of the long faces open (Image 2).
Cut a remnant of fabric big enough to cover the entire outside surface of the box with 5-8 inches of extra fabric on each edge. Place it on a work surface right side down and place a piece of batting just a bit smaller on top of it. Cut slits in the corners of the batting so it won't look bulky and will fold up more easily. Place the box, face side down, on top of the batting.
Starting with the corners, wrap the batting and fabric over the edges of the foam board box as you would wrap a present (Image 1). Pin the edges onto the inside of the foam box with finish pins (Image 2). Since you're pinning, not sewing, you can change the material any time you like.