Indoor window shutters. Two fill a window that is 36.25" (about) across, shutters are 26" high. They were used on my lower windows, as opposed to the entire window. There is an overlap, so you likely would be able to fit a space that is 36" or bigger.
Note - the picture I got off a website. They DO look this, but the window that had them used them only on the bottom half...so you have options. :-) hinges are attached.