From Longman Dictionary of Contemporary Englishsentry boxˈsentry box noun [countable] PMa tall narrow shelter with an open front where a soldier stands while guarding a building
Examples from the Corpus
sentry box• Yet once behind the sentry box, guilt and failure lay stacked on the barrack square.• The club entrance was dark; there was no guard in the sentry box, and the gate was open.• The sentry box was burning on its side from the phosphorus grenade they'd posted into it. 