Which horizontal blinds should I choose for the thermal insulation of a 5m x 1.5m mezzanine?

Hello
I have a question for you. I own a detached house with a mezzanine floor. During the winter I would like to cover it because there is a fireplace underneath. All the heat escapes to the top which means that adequate heating of the bottom results in overheating of the top.
Therefore, I would like a horizontal airtight shutter/blind to separate the downstairs and keep the heated air more at the bottom. The dimension is 5m by 1.5m. any solution?
Regards

In my opinion, there is no point in extending anything as the air will, sooner or later, reach the top anyway. I would rather think about how to change the circulation to change that. There is no drawing so it is difficult to get better ideas, nevertheless....

- you can install extraction units under the ceiling to move the warm air to other places
- insulate the floor of the mezzanine floor to make it less warm
- install an air supply from the cooler part of the room on the mezzanine floor, which cools the mezzanine floor and forces a different circulation.

A lot depends on the construction and location of the mezzanine floor.