How do I give an object a solid exterior but a void interior?

Hi, all!

I'm planning on making a game that involves islands in the middle of the ocean. Of course, when on the island, the player is allowed to walk freely, but not beyond the borders of the islands and into the seas.

I was just wondering if there was a way to make the interior of the island non-solid, but the borders the opposite (Without using an abundance of collision polygon points).

I thought of using containers and adding 4 borders each time an island generates, but that would just be too many objects that will slow the game down for sure since there will most likely be a lot of islands.

I was also wondering if there was a way to invert the collision polygon points. That way I could just invert a square border on top of my object.

Thank you.