How do you create a skybox in Fortnite's Creative Mode without the actual sky showing through?

I’ve spent over an hour trying to figure this out. I know that you need to place a barrier down and set it to the nebula or star field that you want to be displayed, make it its maximum size and have it set to hollow so players can still move around inside. I’ve done all that, but for some reason the color of the island’s natural sky still shows through the walls of the barrier. It’s fine for the blue nebula and star field, but it messes up the other colors, which are the ones I want to use. I’ve had to adjust the fog and lighting settings to make it work, but when I do that the lighting isn’t bright enough.

I tried looking at the Space Showcase island template to figure out how they did it, but I honestly have no idea how they got the nebula to appear everywhere. It’s seems like the entire sky has just been replaced. Even when I fly above the orange out of bounds wall the sky still looks like the nebula. I don’t know how they managed it.

What am I missing here? How to I set up a solid skybox that won’t show the actual sky at all?