I usually have some PVC or Delrin barstock around, which is useful for many things including oddball supports and pushers for case work, making movement holders etc. I'll chuck up a scrap of suitable size and bore out a recess to accept the crystal. You have to creep up on the final diameter so the crystal fits in snugly but not too tight (and not too loose!). There's always a hole in the center to allow pushing the crystal out when finished. Once the crystal is in it will be running very true as you leave the plastic in after turning the recess- this means you can do accurate work even with a wonky chuck. Now, with a very sharp tool, bore the crystal to just fit the ring.
The pics show a tension ring crystal I had lying around, I hope they're illustrative enough.