Gap around curtain wall

Hello,

I am working on a project which has a bunch of curtain walls drawn into a concrete wall. The curtain wall automatically creates an opening in the host concrete wall. The problem here is that the opening matches the exact dimensions of the CW. There should be a an actual 20mm gap around the CW (to easily place the CW in the concrete opening and cover the gap with a filament)

I am looking for a way to create a dynamo script which would create that gap between a curtain wall and the host wall.

Any ideas on how to do it?

I am thinking of a wall void which follows the CW profile or the boundary mullions. But I am not familiar with the particular nodes which would help me achieve the result.

Thanks!