Unable to draw the correct unit cell in CST using AutoCAD

Dear All, I am currently doing a project that requires me to use both CST and AutoCAD. Please refer to the screen shots below for the models that I have drawn in AutoCAD before exporting it to CST. The entire patch is solid which is not what I want. What I need is a gap in between the patch and the outer ring as shown in the picture below. I am unable to edit the structure in CST. Please advice on what I should do on the AutoCAD side to correct the error. Thank you

Have you tried using Boolean operation to subtract from the square with L1 sides? I find CST workspace is quite easy to draft the unit cell and I can export into AutoCAD and duplicate more if I need to.

you can change the material of those part into vacuum as well.