I agree with Mattia, there will not be many other variants. Installing a flap afterwards will be difficult. In my opinion, the right flap is missing here, because it has to solve the odor problem.
I also hope that the fireplace is not used in this form. That is life-threatening. I see the absence of the flap as a major defect and it must be fixed.
Now in winter without a fireplace, I also find that annoying. :mad: