The Bubbling Cauldron had thought a long time about his costume, once he understood that this was a dress up event. Not well versed in spellwork, though with more practice at some of it than his yearmates, he was able to manage a decent enough fog spell that came off the top of his cauldron, though he'd had to seek out some help with the bubbling part from an older student.

Beyond the fog, which he had managed to get to glow a green tint, and the bubbles, his costume was pretty simple. He wore his black school robes and two hula-hoops. One was attached around the bottom of his school robes with a sticking charm and the other was around his shoulders, with black material stretched over it, in the middle of which his head emerged. It was onto this black fabric surface that the bubbling and the fog came from. 

Walking into the masque, his eyes, under a green ski mask that matched the green fog color, went as large as a pair of Galleons. Hogwarts was definitely far cooler than he had ever expected it to be, he thought as he made his way further inside.