The glass item needs to get blocked in so to speak. You can accomplish that by using wedges made from wood or thick cardboard, even though you may have to have several due to the change in shapes of your glasses. I find it much easier to use a bean bag or a soft pillow, which allows you to push the glass into it, conform to its shape, and adjust to any new shape you may need to work on.
-Ruth Dobbins, Professional Glass Consultants
