Shaco "the demon jester" is about as demonic as "Star Wars: Attack of the clones" is about a clone attack.
Also Shaco's hat reminds me of horns to begin with, so that's where the idea for this skin came from.

The demon skin line only has Vi to show, so that's what I use as reference point with options for change.
For example: Pink skin is no problem, but white hair on Shaco just makes him look really old. Therefore I also used Little Devil Teemo as another reference point.

While looking up other Shaco skins I noticed that the Box was literally never used to do something creative with. It's always merely a little mirror of the Shaco skin it comes with.

US forum of LoL where you can support the skin:
https://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/OxXMX1EV-skin-idea-demon-shaco