A procedural skybox shader I made in GameMaker while I've been super into graphics programming
The only geometry in the scene is a cube around the camera. Everything else is shader math.
Loosely based on the default Unity skybox, though it's probably faster to pre-render the skybox, especially for reflection maps.
This is maybe going to be included in the v2 of my Skybox Converter (It maps a 2D Horiztonal-Repeating texture into a rather convincing skybox)