butt hutt
n.— «Some boys end up in unsupervised group rentals they call “butt huts” because of the crowded sleeping, while others live in cars or end up in jail.» —“Boys Cast Out by Polygamists Find Help” by Erik Eckholm in St. George, Utah New York Times Sept. 9, 2007. (source: Double-Tongued Dictionary)
“Butt hut” has also been used to describe an outdoor shelter where smokers are allowed to light up, for example: http://no-smoking.org/feb02/02-13-02-2.html