Doddle ! You should get a tube of black gunk that it thats a seperate part number and there are two recesses in the bits of the spoiler that touch the boot lid to squezze the gunk into. When the mastic has cured , you would need a hammer and chisel to renove it !
just hold the spoiler down with masking tape while its drying and don t be tempted to drive for 12 hours.
Not many on here seem to go for this mod but I think it looks pukka on my stratus grey