never taken mine off, but years ago i robbed one off a 16v in a scrappy and it was just stuck on via 2 clips, thats what the screws go into - they are STUCK to the glass with epoxy or whatever, no other mounts at all.
when i stuck it back on i just siliconed it on and it never fell off not even after numerous beatings in the car wash
bare in mind that the spoiler doesnt do feck all, so theres no load on it to shear it off like there would be if it was larger.