Sealing middle of sliding window with portable air conditioner installed
I had this same problem and ended up using some foam piping I found in the window seal section at the home improvement store. The next best thing I could think of was cutting some foam to the exact fit of the window, compressing it before you open the window, then opening the window onto it, moving the foam into place as needed. If you use sound proofing foam, you might get a noticeable drop in noise. Other than that, I'd buy some good ear plugs.