In my bedroom I have a one hose portable AC unit, because my only bedroom window is a sliding window. Currently, I set up the window kit in the day and take it out before bed. The reason for this is because of the road noise at night from the street the window faces. Picture of window
To seal the window kit I am using foam pipe insulation around the edges, which works quite well. The main source of noise, however, is coming from the opening between the two sections of window in the middle. When the window is closed, the middle frame of the sliding section seals against the other frame, blocking out any noise. When the window is open, there is a gap created (approx 49 1/2" tall, 3/8" wide).
My first attempt at sealing the gap was to use half a section of foam pipe insulation jammed into the gap, which does block out some noise (and outside air) from getting in, but still not to a level where I can sleep comfortably with it installed in the window.
So, my question is what would be a good way to seal the gap?