I would like to clarify things relating to using strings and לבוד to create a wall.
2 points by way of introduction.
1. The acharonim say that a wall created just by לבוד one way (e.g. strings across the wall like I suggested) is called a מחיצה גרועה a weak (for lack of a better translation) wall
2. A succa does not require 4 walls (like for a reshus hayachid on Shabbos), rather it only requires 3, and really only 2 plus a tefach.
Given the above the Magen Avraham (סי' תר"ל ס"ק א) writes that if you are creating a wall(s) with לבוד you need to have 4 walls. In other words , the leniency of of 3 walls only applies to real walls, but if you have a מחיצה גרועה this leniency does not apply and you need to have 4 walls.
This means that if you want to be machshir your canvas succah using string and לבוד you must do this on all 4 walls.
One issue that comes up is what about the door? One wall is not complete because you need to leave room for the door. That would seem to be ok if the door is not large because the wall is majority there and only minority open.
There is another way to do this which creates better walls. If in addition to the strings across, you put string going up and down less then every 3 tefachim the length of the wall, you end up with a real mechitza. The reason being that the strings go both ways (you end up with squares less then 3 tefachim in length and width, שתי וערב) and there is no big gap of space.
In truth, I never did this, but after reviewing the sugya if I was being machshir a succah based on strings and לבוד this year I would do this for my peace of mind. It is not that much work (for an 8 foot wall you need to put up 12 strings) and by doing this you create real walls and therefore only need 3 walls and don't have to worry about the door, etc.