Super Z Outlet Foam Rectangle Blank Polystyrene Sheet Boards for Arts and Crafts Blank White DIY Project, Party Decorations (16 x 11 x 2 Inches) (3)

They usually come with a screen you can use to seal it. If that doesn't fit, get some material (a styrofoam sheet will work best, but you can use a plastic sheet, cardboard, whatever), and cut it to fit the window hole. They have them on Amazon in whatever size you need. https://a.co/d/03vjVfJA