Here's my version of a similar setup, using nylon webbing "slings".... the really important part of using strings or slings is that one must adjust the airplane horizontally so that whatever the longitudinal "datum" reference line is - is parallel to the ground. That is the ONLY time the plumb bob will point to the real center of balance. If you set it up with nose slightly higher than actual reference, the bob will show "tail heavy".... get the tail a little high, and one could be in REAL trouble, 'cause the bob will show "nose heavy", and that could be ugly.

Pics show how I did my Ultimate bipe and a Goldberg Yak...