This is the 6" Vacuum Chuck that I used on the Maple Bowl; the bowl is about 13 inches in diameter. Notice the hard plastic between the two o-rings, that is what leaves the mark. Larger Diameter o-rings might help the problem, but they will compress to seal the vacuum chuck to the bowl and the hard center will be in contact with the bowl.
Enlarge the picture and you can see the mark left by the vacuum chuck, just above the blue tape
I think my gauge is off a little bit, it should read close to 30 inches of vacuum, because I am near sea level.