Sorry for the second post, should have just tested first. Jan, you can do what I just tried. I made the cut out shape first - the shape you want to be the negative space. I worked in the vertex room to create the fire shapes, circles, and bars. Then I added thickness, aligned them just so, and exported or saved as .obj file. Then I imported it back in as one object. Converted it to the vertex room ( another modeler ) and used it as the subtraction for the rectangle shape. It took some playing around but it worked. Following this method, you can cut out compicated shapes. Good luck.

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