How to create an island?

Hello guys! I’m making a simple game and, now, my landscape it’s a “normal” one, surrounded by mountains. I’ve changed my mind and I want to change this landscape to an island, but, how do i do that since the landscape tool have a square shape.
I was thinking in create a landscape and, with the sculpt tool, create a higher part, “cutting” the corners

What I would do would be to push a nice “rim” around your landscape down and then place a big plane with something like water or fog to hide the edge of you landscape (and not make it look like the island just “ends” there).
Keep in mind that if you want your island to end in a very sharp cliff, the textures are going to get stretch quite a bit on that part so you might preffer creating either a softer slope (a beach maybe) or putting some rock meshes on parts of the cliff if it’s too visible.
You can also put some smaller low poly mountains or a large ring with some flat landscape pictures in the far background to break down the horrizon a bit and make it look a bit more natural!