ridleusmc - thanks and you're welcome. You may try hardware clamps, paper clamps and everything else. But at the end of the day I believe you'll find out that paper clamps stick most firmly to the steel structure of the display case than any other things.
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tonka - thanks for your comments! I hope you'll find one. I bought paper clips from a stationery store and glasses from aluminum and glass store. A 3'x4' glass plate can be custom cut into 6 plates of 33x36cm.