The one I made up out of hex stock - while waiting - has a hole running through the center... I didn't use a screw to tighten it, I am relying on pressure from the vise jaws. Simple enough to tap the coin out with a Delrin or wood dowel. Maybe that'll help?
Brian P. Marshall
Stockton Jewelry Arts School
Stockton, CA USA
209-477-0550
instructor@jewelryartschool.com
jewelryartschool@aol.com