(Solved) Creating 'negative' shapes in Gerber output

[angust_uk]

I'm trying to create a Gerber in AD20 to be used for a photolithography mask, and the features are to be inverted (i.e. the bits of interest need to be 'clear' and the background needs to be 'copper' (hopefully the image below makes more sense; left is imported DXF in Altium, right is the desired pattern). I'm struggling a bit to figure out how to do this; I can convert the large circle to a copper area (Tools -> Create Region from Selected Primitives) but I cannot figure out how to create a cutout in this region for the 2 long features.

I've also tried to turn it into a polygon so I can create polygon cutouts, though I can't seem to convert the large circle (imported from a DXF) into a polygon successfully (when I try 'Create Polygon from Selected Primitives' it just gives me another un-filled circle with a slightly wider border). Is there a method I'm missing to do this?

Edit: I've managed to convert the circle to a polygon (I'd forgotten to ensure its properties were set to 'Solid') but I still can't seem to create the cutouts in it. I've tried Tools -> Convert to Cutout with no success so far. Again, any hints on 'subtracting' an imported DXF shape from a polygon would be appreciated!

[angust_uk]

Managed to solve it eventually! In case anyone stumbles across this, convert the DXF shape to a region, then set the 'Kind' property to Polygon Cutout.