Next time before you apply a stick-on type of UTH pad, be sure to clean the glass thoroughly, then again with isopropyl (rubbing) alcohol, to remove any film left behind- that can definitely cause a "failure to adhere". But now it's too late for that- as others suggested, tape the edges.
OR, depending on how much space is under your tank & what it's sitting on, you could put a ceramic tile underneath the UTH to fill the gap- & so the UTH won't have anywhere to fall.
Be cautious- you can crack the glass if the tile you pick is too thick for the gap & you quickly set the heavy tank down on it. Tiles come in different thicknesses- and you can buy a few random leftover tiles very cheaply from businesses that install flooring. Tile works great also because it functions as a "heat sink"- holding the heat a while like rocks do, if your power should happen to go off for a while. I keep a few tiles on hand, besides those I'm already using under tanks.
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