Slight catch. if you suspend a rope pendulum from a horizontal string, if the string flexes at all, it will essentially act like a longer pendulum perpendicular to the string (where the string moves) than it does along the string. The result is that the pendulum's swing has a different period along either axis, and it will trace out a lissajous pattern, not a straight line in its swing. And that sort of wrecks the idea of a focault pendulum.

Oops!

Better make sure your cable is REALLY tight.