Randy -- When I worked in a furniture mill, we had a line of rustic furniture that was made from knotty pine. We used hot glue, applied to the back side, to keep the knots in place. Most hot glues are flexible, so the hot glue worked well given the different amount of contraction/expansion you'd expect between the knot and the surrounding timber. Hot glue is also quicker than mixing up epoxy and is far less expensive.
HTH