We cut Reflectix, which you by by the roll at Lowes or Home depot, to conform to each and every window. We place it in the windows, tuck the corners and then pull down the day\night shades to hold it in during the heat of the day. We have taken temps with our infrared thermometer with and without the windows shields on a 100 degree Central Texas Day in full sun, and it is a good 15 degree difference, just using the Reflectix. We have now cut it for the sky lights in the kitchen and Shower and you can tell more difference, and it makes it pretty dark on a full sun day.
We use it also in the winter in the windows to keep heat in and it adds about 15 degrees back to the rig in 20 degree 20 mph wind. Found that out on our Christmas Trip to Kansas city! We are a believer in the Reflectix solution.