Step 2: Cut window insert panel to size
Cut a piece of rigid insulation or wood to fit in your partially opened window. In the second photo, it's the light blue piece that's jammed into the window. I used foil-covered 1/2 inch thick polyisocyanurate insulation board; this is much quicker to work with than wood, and is a better insulator. Avoid the polystyrene foam insulation board if you can, as it'll make a mess.