Cooper, the hex nuts worked better than I expected, but the size was a good match for the stock so it helped center the round stock. The set screws keeps the stock from rotating inside the nuts. And, since you have to release from the vice and rotate, you have to use a stop or something to index the part for the subsequent operations.
Short version of this story, I ended up buying a spin indexer and 5C collet set!
It is much quicker since you don't have the vice issues to deal with.
Jim