Tatty box with door in one side and corrugated metal roof?
They can fit through a surprisingly small door. My husband made one for mine, door was about 2' square. But they could fit into my hen house pop hole, which was something like 20cm square, maybe a bit less!
If I had them again, I would make a small pop hole door like this, to keep as much straw in, and keep rain out.
I'm sure you know, but I wouldn't be in a hurry to keep a gander again if I were paid! He had a lovely goose mate, but was so aggressive in spring time (Feb-June) that he would hurt the hens. He would pick them up and shake them, feathers everywhere! He ripped the back off one. He had to go.
If I had no smaller animals or children it would be fine, as they are otherwise quite delightful.