Well, perhaps Harry could be building the boat to escape from some new danger in the wizarding world. Maybe he's on the run from a new dark wizard and needs to cross a large body of water to reach safety.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
Dumbledore might help in a spiritual or advisory way. Even though he's passed away, his wisdom could be passed on through memories or some magical means. Also, Hagrid could be a great help. He could assist in building areas suitable for magical creatures with his love and understanding of them.
In a fanfiction, Harry Potter could be in a situation where he needs to recruit players with strong muscles for the Quidditch team. He might search through Hogwarts to find students who have been secretly training and have the physical strength required for the tough positions in the game, like the Beaters.
Ron could also be a great ally. His loyalty and courage would be important. He might be in charge of security, using his skills in wizard chess to plan out defensive strategies. And of course, the Weasley family in general could offer a lot of support, with their various skills and connections in the wizarding world.
Maybe an old wizard who has been secretly studying the combination of different magical universes for years. This wizard could have made some initial attempts at building a lightsaber - like weapon in the past and can share his experiences and mistakes with Harry. He would teach Harry about the balance between the light and dark side of this new form of magic, just like the Jedi teachings in Star Wars.
Well, perhaps Harry uses his magic to shape the planet. He might create unique magical creatures and ecosystems. For example, he could make a forest where the trees are made of living fire that doesn't burn but gives warmth and light.
Maybe Harry builds a magical trunk that has special compartments for his school supplies, like a self - organizing quill holder and a spell - protected space for his invisibility cloak. It could also have a hidden entrance to a small room where he can study privately without being disturbed by his roommates.
Maybe Harry Potter builds the fortress to protect Hogwarts from a new dark threat. He could use his magic skills and the knowledge he has gained over the years to construct an impenetrable fortress with magical defenses like anti - apparition wards and powerful spells.