No matter what gear you plan to carry on your roof, you will need a multi-purpose roof rack base system. The Thule 400XT Aero Complete Roof Rack Base System will fit your 2008 Honda Fit Sport perfectly. The Thule 400XT Aero system includes four 400XT Aero feet, four FitKit 2180 Clips (specific to your Honda Fit Sport) and two square load bars The 50" length load bars are the recommended size for your 2008 Honda FIt Sport rooftop.
Once you have a Honda Fit Sport roof rack multi-purpose base system installed, you can then mount a cargo luggage box directly to the base rack load bars. I highly recommend going with the
Thule 603 Ascent 1500 cargo box. Connects directly to the Thule 400XT Aero base rack bars. The Thule 603 AScent box is waterproof, includes locks and has plenty of cargo volume to carry luggage gear etc. Also looks ideal on your Honda Fit Sport.