If you have problems with your field points coming off. Try a small lock washer. It's weight is minimal and will help keep your heads from unscrewing. If the washer has a lip that is more then the field point, nock it down with a cheap mill file while it's on the arrow.
If these arrows are dedicated to being pratice arrows, you could always take a drop of glue on the point where it meets the insert. Mind y ou if you do this it probably will never come off!!! Epoxy would set it in the insert forever. Though with some judicious heating you can pop epoxy, I'm not sure if it would ruin the insert or arrow first though.