i cant say for the
ibiza, but the splitter is fairly flexible
i know those who have put them on leon MY03 bumpers just sliced off a bits of the 'clips' so that it fitted fluch with the bumper, then drilled small holes through splitter and understide of bumper and used self taping screws.
or just ordinary screws.
short fat ones would be best.
easiest way would do to fit the splitter using stikcy tape or something like that to get positioning correct, then drill and screw insitu.