I like this - it could even work as a form of one-time-pad, which is nearly unbreakable.
Another feature would be the knots and such must be unraveled to be read, making it almost impossible for the message to be intercepted, read, and then passed on. This is a big deal since not receiving the expected message, or one with signs of tampering can warn someone. Go to Comment
I think this is an excellent idea, though it depends upon your magic being loose enough to accept this. Some fictional (aren't they all) examples of circle use are so exacting as having a hair drop across a line is sufficient to blow the protection. With such standards, a carpet would be play Russian roulette with 5 rounds loaded.
For D&D though, or lower-stakes magic, I see this as perfectly fitting. Go to Comment