Render Skysphere on a plane?

I am currently working on and effect in my game, where “the sky opens”.
Right now I tried using a render texture on two planes, to make it look like the.
BUT right now it looks kind of ■■■■■■, as you can see on the screenshot.

How is there a way to do this without using a Scenecapture2D + Rendertexture?

The simplest way to achieve this would be to create a custom mesh that has a sky texture attached to it. Your mesh would be a sphere that was animated to open as you desired. When you wanted it to open, you start the animation. When you wanted it to close, you simply reverse the animation. If your wanting s dynamic sky the render to texture and have to be used to get the current sky mapped to a texture.