• As you guessed;
• Draw Edge Cutout on Edge_Cut layer.
• Draw same shape on a Silk layer (doesn’t really matter as you’ll move it to Copper layer). Use the Graphic Polygon tool and Offset the outer loop.
• Double-click the Graphic Polygon and select desired Copper layer to move it to.
Quick example, below… with it on the Bottom Cu layer. I didn’t fuss with exactness/dim’s…
Thus; a PCB with and edge cutout that is typical (not any different than any other PCB with a particular shape). And, a copper fill or, in this case as I did it, a Graphic Polygon (which is what is used for making a custom Copper fill unless loading a custom graphic…)