Maybe it was given to him by a mysterious stranger. This stranger could be someone who has been observing Harry from afar and believes that he is the only one who can handle the power of the hand cannon. The stranger appears out of nowhere, hands it to Harry, and then disappears again without a trace.
A corrupted Naruto fanfiction might show Naruto or other characters using dark magic or evil powers in a way that goes against the original story's concept of chakra. If the story is full of plot holes and the characters' actions seem to have no logical basis, it could be a sign of a corrupted work. For instance, if Sasuke suddenly teams up with the villains for no good reason. Additionally, if the fanfiction has a lot of OOC (out of character) dialogues where the characters speak in a way that doesn't match their personalities at all, it's likely to be corrupted.
One sign could be that Gaia always tries to keep Naruto close to her. She might constantly be around him, showing up wherever he is. Another sign could be that she gets jealous when other characters interact too much with Naruto. For example, if another female character shows interest in Naruto, Gaia might act in a way to drive that character away.
One popular 'Harry Potter uses sleight of hand' fanfiction might involve him using sleight of hand to perform magic tricks in a Muggle - style way during a Hogwarts event. It could add a unique and fun twist to the magical world we know.
This type of novel is not popular in real life because it violates traditional moral values and social values. In real life, attacking and receiving should respect and support each other instead of using and dominating each other. The novel is a fictional art form that can express the author's imagination and creativity, but it should abide by the basic principles of fiction and social morality. Therefore, I would not recommend this novel.
Might Guy is also a top choice. He is Lee's teacher and has mastered the same style of taijutsu. His experience and his ability to open the Eight Gates make him an extremely powerful fighter in hand to hand combat. In fanfiction, he could be shown in epic battles, using his full power against strong adversaries.
I'm not sure of the exact titles, but you can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Search for 'Harry Potter uses sleight of hand' and browse through the results. You're likely to find some creative takes on this concept.
Master Hand could be an ancient being that has been observing the Naruto universe from afar. Maybe he's a sort of guardian of the balance between different worlds, and when Naruto's situation threatens to disrupt that balance, he steps in.
One interesting scenario could be a fight between Naruto and Sasuke where they rely solely on hand to hand combat skills, without using any ninjutsu or genjutsu. It would be a raw display of their physical strength, speed and martial arts training.