How to make 6 skybox textures from skysphere?

I have one sphere model and some skybox texture(one texture to one sphere). But now I want to change it to 6 textures skybox which can set in RenderSettings because my reflective water cannot see the sphere model.

Here is the question:
Is there any good way to separate my sphere texture to 6 parts that can fit the RenderSettings’s skybox?

Get this tool: Cubing The Sphere