M. In my opinion, this cover does not work and is not worth the money. I quickly disposed of it.

I can't remember exactly how big the thing is. The uncovered gap will probably be relatively small and can therefore be easily neglected. (It's just a matter of not blowing all the warm exhaust air back in again. Besides, it is not necessary to cover the entire perimeter, but only 3 sides).
If it is not a permanent installation (it probably won't be, you will probably want to close the window from time to time), you can also help out with a towel. We did this several times because we didn't have enough Velcro strips for all the windows we used.
Conclusion: Try it with one part, in an emergency you can always order a second one.

Thank you for the answer