Rashi learns that the prohibition of making an ohel only applies to using a curtain as a roof, but using a curtain as a wall is permitted. Rashi seems to hold that even if the mechitzah is functional to permit carrying, it can be made on shabbos. Tosafos disagrees with Rashi and writes that just as we find in the gemara that the third wall of the Succah which will be machshir the Succah cannot be made on Shabbos, any mechitzah hamateres cannot be made on shabbos. Tosafos holds that a mechitzah that is meant to permit carrying is also a mechitzah hamateres and cannot be made on shabbos. Based on Tosafos, the Rama 315:1 writes that one cannot make a mechitzah in front of a lit candle to permit having relations because that is also a מחיצה המתרת and is assur to make on shabbos. There is a gemara in Beitzah that seems to contradict this. The gemara 22a seems to hold that it is permissible to build a wall to block the light of a candle. The Taz writes on Hilchos Yom Tov (514) that if it is permitted to put out the candle, it would also be permitted to make a wall. However, the Chazon Ish (52:14) disagrees with the Taz. Even if it is permitted to put out the candle, it should be forbidden to put up a curtain. It would be similar to saying that since one can eat in another Succah, they can put up a wall in this one. Why would the ability to put out the candle permit the making of the wall! The Chazon Ish explains that the answer is as the Magen Avrohom says. All one needs for a candle is to block the light, therefore the mechitzah that blocks a candle doesn't need to have status of a mechitza so long as it blocks the light. The mechitzah would be the equivalent to the candle going out which permits having relations without constructing a mechitzah hamateres. Based on the Magen Avrohom it comes out that if the mechitzah blocks out the light it would not be a mechitzah hamateres and be permitted, but if it doesn't block the light then it would only work because it is a mechitzah and therefore forbidden. The Chazon Ish continues that the Magen Avrohom also says that if one doesn't tie the curtain on the bottom, it will not be considered making a mechitzah since it can blow in the wind but the Chazon Ish disagrees and says that since it is being built in a house where there is no wind, it qualifies as a mechitzah even if it is not tied down.