Usually these cloths fit, which you attach to the window with Velcro. They have a zipper where you can make an opening where the hose fits through. However, these cloths are relatively expensive. They were available in 2018 for 19.95 but now they easily cost 39 francs or more.

Sichler exhaust air window seal for mobile air conditioners

An alternative would be a plexiglass pane that fits into the window, which can then be locked in place at the side and into which a hole is cut for the hose insert. However, you would have to make it yourself.