You could super glue some semi-powerful magnets in place of the suction cups and place magnets on the outside of the tank. This is how the hydor powerheads mount. If the powerhead moves to easily, glue some rubber to the magnets so they have something to grip too, this may also be good to prevent the magnets from scratching the glass.
Below is a hydor koralia powerhead, I circled the magnet, one side attaches to the powerhead and the other is on the outside of the tank. The rest is a suction cup, but it is more for to keep it from sliding down then keeping it attached to the glass/acrylic.
I hope this helps, I didn't do the best at explaining it.
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