What things can be used as "Sutra" while there isn't any wall/pillar or any other object in front of us?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Praying to a Sutra (an object placed in front of the praying person to prevent people from walking within his praying area) is a Sunnah. The Prophet 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 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."

Therefore, if one cannot find something to put as a Sutra, then he should draw a line on the ground and this would serve as a Sutra. Abu Hurayrah narrated that the Prophet 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]

So, in our view, if one prays on a rug, the end of the rug serves as a line when there is no Sutra, or when one is unable to take a Sutra, as the rug serves instead of a line and instead of a stick when one is unable to put these in front of him.

Finally, in case one draws a line, it should be drawn horizontally from the right to the left side. However, Imaam Ahmad is of the view that the line should be drawn arched like a crescent.

Allaah Knows best.