Wiktionary
n. (context chiefly British English) A separate compartment of a building in which livestock are free to move about; especially a form of stable for horses
Usage examples of "loose box".
Rejoicing that he could get her that wet so quickly after five years of marriage, finding it always as exciting as pulling a groom in the back of a loose box for the first time, Rupert moved his fingers upwards as Taggie's hands fumbled with his zip.
The shed, made of weathered wood, was built like a single loose box, with a window and a split door, and I imagined it was there that Mickey had spent much of the three days he had been away.
Wrey raised a finger and four men in Army uniforms detached themselves from various walls and stepped forward, halting in a loose box formation about Jonny.