Yeshayac, If I understand you correctly, you are wanting inverted corners with an inset stroke and easy edits to modify. Here are the steps I used. It’s just one rectangle. With your rectangle tool, draw a rectangle. (shown in brown) In your Transform panel, select inverted corners. Type in the radius you want. I choose 24. While still selected, Appearance Panel, Add a new Stroke, located in the flyout menu, shown in cyan here) Give that cyan stroke an offset effect. (Effect>Path>Offset. Here I used -8) OK, say you want to modify. No problem. Everything is still live. See the screen shot below with the new settings. K
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