Super glues are really good for what I think you are saying as they are not toxic after just a short time and while they hold things from pulling apart, super glues are very easy to break the bond if we work them sideways. So that leaves it easy to pry something like plastic that bends off a glass and then scrape any remaining residue off with a razor blade scraper.
Thinking of sticking a flat plastic to the glass side, I would drain down to leave space to work, lay the plastic in place and use one of the thin super glues to add dots to hold them together. Point to remember are that the glue does turn white and really shows big time and we need to give the joint a bit to cure before adding the tank water. The drying is not a big things as super glues react using moisture rather than dry so a wet item from the tank is ideal for them to cure about as fast as we can work.
Neat use for super glue is to pull a rock out of the tank, dab on glue and stick a plant in it and it cures so fast I can add it back almost immediately! Silicone can work fine but may need 24-36 hours to let the toxic parts gas off. I judge them as good when I can't smell the fumes.