Create inverse object

I am trying to create an interior shell to be 3D printed, clear. This will be inserted into an elaborate dome frame, with window openings. I have the frames solid, and created an interior ‘skin’ to form the bases for the ‘windows insert’. problem: windows are holes, how to invert, to create the windows as surfaces. Tried the molding methods to no avail. I can include a photo.

Share the .skp file so we can see what you’ve got. Very likely you can model the window “glass” as a solid rectangular prism and trim it with the frame object using the Solid Tools.