How to Integrate Current Skybox (Skybox Shader textures)?

Previously we all use a material with "RenderFX/Skybox" shader, and assign 6 textures(front, left, right, back, up, down), then use this material in Project Render Settings.

The example ovr skybox in tuscany demo use a custom mesh, with 5 textures and a symmetric look. But that's not what I want while I want to import my old skybox textures and make the same appearance.

So how to do this? I do not know how to use model software so I can't make a custom cube mesh for need. Can someone help? I think this will help many people too.