If you decide that you want to build such a contraption you need the following materials and tools:
• A sheet of plywood that fits into the window frame in question, 5mm thick works good for me.
• A sheet of plywood that is about 1.5 times as long as the frame to cut the fins, I've used 3mm thick material.
• Some 20mm x 30mm scrap wood. (The actual dimensions are not very critical, however, they need to be at least 4 times the length of the window to from the rails for the fins.)
• A bunch of PC cooling fans, the bigger the better. I've chosen 100mm ones.
• A DC supply that is capable to drive the fans.
• Wood glue
• Weatherproof paint
• Wood screws (25mm).
• 4 machine screws (about 6x50mm)
• 4 thumb nuts matching the machine screws
• The only no so common tool required for this built is a hole saw that matches the fans.
• Power drill
• Table saw or some other kind of miter-saw
Step 1: Layout the Window Insert and Cut the Holes for the Fans.
This first step is crucial, so I would recommend to spend some time on this step.
Fist of all measure the window frame to determine the required size of the insert. This part should fit into the window frame with about 2mm gap all around the edges. When inserted, this part should rest securely against the window frame's foremost rim. I would suggest to cut the plywood to size and then try to insert it before proceeding.
When the insert is cut to size, layout the fans evenly across and mark the cutouts with a compass. Keep in mind that the screws that hold the fans in place are also used to attach the strips that hold the fins on the other side. It us a good idea to try that layout with the window in question before cutting out the holes for the fans. Some element may conflict with elements form the window frame.
The fiddly part is over now, so cut out the holes using a hole saw. If you don't have one also a jig saw would do.
Step 2: Pepairing the Insert
Deburr all the edges and prepare the surface of the ply to take on the finish you have chosen. I would recommend to sand it to 180 grit or so.
Step 3: Preparing the Strips and Cutting the Fins
In the next step the thinner ply is cut up into strips for the fins that will protect the fans and the interior from rain. If you want them to cover the openings perfectly, make them 1.41 times longer than their spacing (assuming installing them at a 45° angle). Don't forget to cut a longer one for the very top, the size of this one depends on the hight of the stips you use. This one needs to be (1,41 * hight of the strip) - (depth of the mounting cut in the stips) longer than the other ones. Then cut slots in the strips at the chosen spacing and a 45° angle. This slots need to be as wide as the ply and are perfectly consistent if you clamp all the strips together while doing so. Don't forget to miter also the ends of the strips at a 45° angle.
After cutting all this up, sand all the parts to the desired finish and glue up the strips to line up with the mounting holes of the fans.
Step 4: The Mouning Brackets
In order to attach the insert to the window, we need some kind of retaining device. I went for simple blocks with cutouts that grab the window frame's outside and clamp the insert to it. This may sound a bit sketchy but have a look at picture one that should make it clearer. Picture 3 gives you an idea of the final bracket seen from the outside.
In order to build such brackets we need to cut up some scraps to get some blocks of about 70x20x30mm. The absolute dimensions are not critical, use whatever you have. I used some leftovers from cutting up the strips.
Step 5: Forming the Clamping Face
This step depends on your actual window frame. Cut out a notch that mimics the form of the window frame's foremost rim, so that the bracket is flush with the insert when installed. This is again a bit fiddly.
After you are done with that, drill a hole through the center of the bracket with an appropriate diameter to take whatever machine screw you have chosen to use.
Step 7: Cutting Slots for the Clamping Brackets
In this step drill two holes in the insert for every bracket you wish to install. The outmost hole should allow the bracket to grab the window frame. The inner hole should allow the bracket to retract that much so that it does not interfere with the frame. This position will later be used for installing it. After that use a jigsaw and a file to form a slot from this two holes.
Step 8: Put on the Finish
This thing will get to experience some nasty weather, so better put on a weatherproof paint. You will earn some additional stile points if the color matches the windows.
Use the slots for the fins as a stand when painting them but do not install them at this point. They may interfere with the next step.
Step 9: Install the Fans
Now just install all the fans and wire them up in parallel. At this point you have to decide whether you want to force air inwards or outwards.
I went with having all fans installed so that they force air out of the window. Considering that this insert is intended for a window close to the ceiling (where hot air gets trapped) and I don't want to suck in bugs and dust from outside.
Keep in mind that your DC supply must be able to deliver the cumulative current of all fans. In my setup I run the fans only at about 70% of their nominal voltage in order to get quieter operation. Having a bench power supply at hand is very neat to find the sweet spot between having not too much noise from the fans and still getting satisfactory airflow.
Step 10: Installing the Fins and Brackets
In this step you want to install the fins using short nails, a nailgun or/and epoxy glue. Start from the top and work your way down till all fins are in.
After that add the brackets and add the thumb screws. Fix that brackets in the installation position and you are set.
Step 11: Admiring Your Work
Now is the time to admire your work and have one of your favorite beverages ;-)
Step 12: Installation
Depending on your stile of window frame you want to remove or simple open the window. Since the brackets are retracted, nothing should interfere with the window frame when installing it.
After everything is in place, loosen up the thumbscrews and slide the brackets outwards. As you tighten them again, they should grab the window frame and hold everything in place.
Step 13: Enyoing Cool Air on a Hot Summers Day
After you installed the insert in the window you simply switch on the DC power to ensure air circulation throughout the room. Check the thumbscrews every now and than to be sure nothing gets loose and falls down on you.
Adding some automatic control to this device based on in and outside temperature could be easily achieved using some lines of Arduino code and a suitable transistor.