goop is a stupid strong rubber cement,and I love it, the only thing I would fear is that the fumes would cloud up clear plastic.
usually those speakers are held into place with little metal tabs and a hole drilled in one end to screw them down, you could make some easily out of a soup can ... another thought would be silicone caulk, since its often used on plexiglass I would guess it would not fume up the plastic