I actually attached a handful of radiant heat panels to glass tops I had cut for exo terras, so it is pretty easy to do. I drilled the attachment holes with glass bits and used bolts and wingnuts to hold it in place. Now with that said, I don't think I'd want to do that in a jewelry display case as I don't know what type of glass is used (probably standard plate glass but still). I would probably instead build a support structure to attach the panel to.

Another option would be to look at kane heat mats.
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Reptiles Make The World Go Round.