Decommission and seal roller shutter boxes

Hello,

we will most likely install [Vorbaurollläden] and deactivate our current boxes - which are currently equipped with manual devices.

Do you have a good approach / idea on how I can best seal and close the deactivated and emptied boxes? I do not want to create any thermal bridges or the like.

Thanks in advance

Insulation foam?

Jehovah!! ;-)