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Don't assume they are all bad. That seems unlikely, doesn't it: That everything you get from the glass shop is bad? Glass will deflect. Odds are the aluminum bed is bowed and you're clipping the glass corners to the bed, right? So you're forcing the glass to bow to match the shape of the bed curve. You need to shim between the bed and the glass to fill the gaps but keep the glass flat - plenty of videos on doing that.