Can't apply Skybox material to Skybox type variable

I've been messing around with making skyboxes by script but I can't apply a skybox type material to a skybox type variable in the project view. Here is how I am creating the variable:

public Skybox sky;

I can't apply a material to this that is a skybox, and I can't access the textures on the material to make a six sides skybox. What am I to do?