Once you have your materials, check that the window frame is square. If not, at least make sure your window is structurally sound—it can be trimmed square before construction begins. The outside dimensions of the main frame should be 1⁄4'' smaller than your window on the front and sides; the back must be flush to allow hinging. The resulting overhang helps shed rain and makes the window easier to grab and open from any direction.
The window is angled to increase solar gain, so the front of the main frame is half as high as the back. Dry-fit (put together without attaching) the main frame to match your window size, then join them into a frame with 2 x 2 corner blocks, #10 x 11⁄2'' wood screws and weatherproof glue. Use carpenter’s glue with a type II rating or polyurethane glue. The bottoms of the corner blocks on the main frame are raised 1⁄4'', while the tops are flush, allowing the booster frame to nestle under the main frame and lock there.
Using scrap left over from the front panel, cut two lid supports. Position the pivot holes for the supports far enough forward so the ends of the supports extend over the front of the frame. This allows you to prop the window open slightly when just a little ventilation is needed. Add chains to stop the window from tilting back and damaging the hinges. Hook-and-eye latches will keep the wind from lifting the glass and breaking it.
The booster frames should be the same length and width as the main frame but without tapered sides. The corner posts are cut the same length as the width of the sides of the booster frames, but installed so 1⁄4'' sits above the top of the frame, which fits into the 1⁄4'' gap at the bottom of the main frame and leaves a 1⁄4'' gap on the bottom to fit an additional frame if required.