the answer to this is in the last part of the question you have asked. placing a barrier (sutra) is the sunnah. there are numerous ahadith about placing of the sutra for the person who is praying alone. ill try to explain a little more, insha Allah.
The Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) said: "When anyone of you performs a prayer, he has to place a Sutra and should be close to it." [Abu Daawood and Ibn Maajah]
If one cannot find anything to put as a Sutra, then Imaam An-Nawawi may Allah have mercy upon him said: "If one does not find a stick or the like, he should gather some stones or soil or even his belongings in front of him, otherwise he can put a rug or draw a line."
Abu Hurayrah may Allaah be pleased with him narrated that the Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) said: "When anyone of you performs a prayer, he has to place something in front of him, but if he does not find anything, he has to put a stick, and if he has no stick with him, he has to draw a line and he will not be harmed by anyone passing in front of him beyond the Sutra." [Ahmad, Abu Daawood, Ibn Maajah and Al-Bayhaqi]
from these and with many other authentic narrations it is certian beyond any doubt that we have to put up a sutra when praying alone. i find it worthy to mention the following hadith because sometimes in such small spaces when shared by females, one might find the other sleeping when praying. (this hadith is relevant if the qibla is towards the direction of the person sleeping, like you have mentioned).
At-Tabaraani narrated in al-Awsat (5246) that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: "I have been forbidden to pray behind people who are talking or sleeping." Classed as hasan by al-Albaani in al-Irwa’ (2/96). Also narrated by Abu Dawood (694) from the hadeeth of Ibn ‘Abbaas as follows: "Do not pray behind one who is sleeping or talking." Classed as hasan by al-Albaani in Saheeh Abi Dawood.
may Allah show you mercy and guide you to the straight path.
Allah knows best.