A very simple and small building made from pieces of wood, metal, or other materials. (Cambridge Dictionaries)
A roughly built hut or cabin. (Oxford Dictionaries)
Shack up. Move in or live with someone as a lover. (Oxford Dictionaries)
A room or similar enclosed structure for a particular person or use. (Merriam – Webster)
A rough cabin; shanty. (Dictionary.com)
A shack is a simple hut built from tin, wood, or other materials. (Collins Dictionary)
The shack at Hinkle Farm.
Shack in the Edo-Tokyo Open Air Architectural Museum.
Tree house, Mississippi River in Trempealeau