Flipping projection matrix problem: It flips everything but the Skybox. How do I fix this?

I am flipping my camera projection horizontally and vertically. by using the following line in the OnPrecull()

camera.projectionMatrix = camera.projectionMatrix * Matrix4x4.Scale(new Vector3(-1, -1, 1));

It flips everything but the skybox, whether it be Marmoset or Unity’s own skybox. How do I fix this?

The simplest thing is probably to take your skybox art into Photoshop or a similar paint package and flip it.