Prefabricated memory plastic cubicle
From Halopedia, the Halo wiki
Prefab Memory-Plastic Cubicles are small structures used by UNSC soldiers in combat operations or in Firebases when there is not any better source of shelter. They are large enough to house one man with accomidations. They are not used very often, as other prefabricated structures are usually brought to the battlefield when the UNSC Marines and ODSTs are better prepared to engage the enemy. According to the Master Chief, these Cubicles replaced the "archaic" concept of tents. Components for the Cubicles are apparently stored in each Human Entry Vehicle.
At Alpha Base, many of the ODSTs used these Cubicles as lodgings because there were not many other accomidations they could take with them from the Pillar of Autumn.