147 Roof Bars

Hi, I have a 147 and I'm after some roof bars to carry my bike, (don't worry they won't stay on any longer then needed). I'm really keen to get the genuine Alfa bars but at £183 these are out of the question. I've seen some on eBay, however they are for a 5 door and I have a 3 door. I'm pretty sure they the back one is different, Does anybody know how I could convert the back one to fit?