French doors cavity gap

[speccy]

Hi we just had some french doors installed in a room was previously a window so new 'aperture' cut to fit.
Thing is the external frame sits in outside skin ie brickwork. We now had a big gap on the inside where cavity is before internal blockwork and screed starts.

Can we board over this gap or put celotex/foam insulation in there first, does it have to be wooden board is there a specific piece i can put in there? I want it to be low profile so the carpet continues from screed level to frame.

Thankyou hope i am making sense.

 

[crank39]

You generally don't fit them retrospectively but could you fit a cavity closer, don't know how big your cavity is but I think eurocell do 50/75 and 100mm