Most bike racks (such as Paddy Hopkirk) designed for hatchbacks will fit the Coupe. I tried the high level mounted (keeps your number plate visible for legal reasons) 3 bike carry from Halfords. But the bike acted like a wing and lifted up!!! Pretty scary when you've got a £2K mountain bike!!! Took the bike rack back.
I now use Fiat roof bars and Barracuda racks manufactured by Mont Blanc. I'd highly recommend the roof bars and I'm sure that roof bar racks by Thule and other manufacturers will perform as well as the Barracudas I use. The maximum weight specified for the roof bars is 25KG. However I often carry 2 bikes up there totalling at least 30KG (one is my mates Santa Cruz Bullit downhill bike and it's pretty weighty) all this without a problem. The clamps that attach to the roof where the doors close, can mark if you're not careful though.
Unfortunately Fiat don't make roof bars for the Coupe anymore. I got mine from eBay for about £50. The Barracuda racks cost about £70 each so it's not a cheap setup, but is very stable... just don't look up through your sunroof if you've got one and you're doing 80mph
Can be a bit scary!