77seriesiii, ordinary 2mm glass will do the job for your 11x14" contact printing frame. I have one from Bostick and Sullivan (same size as yours, actually oversized for allowing brushmarks and white space around the image) and use it for different alternative processes w/o troubles. I broke its glass twice before, and I just replaced it with ordinary glass from glass shop w/o any adverse effects. (Have them file - if that's the correct term for glass - the edges and corners to eliminate the risk of self harm while handling it!) I always pay attention to keep a spare glass in case of an accident... I sometimes see Newton rings, but they don't seem to register in the prints. (Maybe UV light - which is what I use for exposures - is less affected from newton rings...)