What clamps did you use and how did you clamp it?
I borrowed a pair from a neighbour. I think they were by a manufacturer called Irwin - "Quick Grip" clamps which you can get at Screwfix. I wrapped the heads in cloth to avoid scratching the spoiler/boot. Tightened to a reasonable level and left overnight - one end on the spoiler itself, the other end in the recess where the number plate is.