Atleast 3/4 inch. I still think a rim should be used, glass and silicone are strong but the weight and pressure could eventually lead to failure. I know all to well. Was once lifting a 3000 glass piece with a crane at work, it was a insulated piece, silicone failed from weight and piece sheared off. Got us scared, which was a good thing because a week later the crane snapped and 6000 lbs fell 3 feet from us where normal we would be standing a week before. Moral of the story, be careful, sometimes things don't work as expected.