The problem with my tensioner is, if it is not tight enough, you have to turn it a full 360° to the next stop. It would be better if you could tighten it in 180° increments to adjust for more optimum tension between the blade and the arms. That design change would require the slot in the paddle to be extended to run the length of the paddle.
I'm using waxed linen twine in my bowsaws. It has held up very well for several years. I originally tried braided Kevlar (mostly as a joke, since it should be obvious that it would be orders of magnitude stronger than the wooden arms), but it is too brittle, and eventually failed, one strand at a time, wherever there was a sharp bend.