Halloween // 3 year old wants to be the castle from Howls Moving Castle...help!

Twin A, 3.5 years old, wants to be Howl's Castle from Ghibli's Howl's Moving Castle. He's been pretty consistent about it for months, so I'd like to get started constructing as soon as possible because my daily free time is fairly limited. I'm up to the challenge and can follow tutorials competently but am not sure where to start here. Any ideas?

He also turned down being Calcifer and riding in a wagon decorated like the castle. He wants to be the ding dang castle itself.

His brother wants to be a "regular" witch (he was a witch pirate last year). I asked if he wanted to be Howl, but he said no ¯_(ツ)_/¯

Thanks!