I've tried yucca and yucca fibers will work fine for a 40# bow. The string doesn't need to be thick: just over 1/8" should be fine. Just remember that the string is only as strong as its weakest spot...so consistency is critical.
If you wind the fibers tightly while twisting, you don't need any "natural glue" to hold it together. It will fail sooner, though, than a sinew bowstring.
Yucca fibers that are harvested green and then dried are stronger than fibers harvested from a dead plant.