Bontrager (Trek's in-house accessory brand) makes a nice "
lightweight rack" for road bikes and the like; however, if you do indeed have a carbon 7.7 FX, then it wouldn't be compatible. The current 7.7 FX basically has the same frame as a 3-series Madone road bike, so you might need some sort of rack which clamps onto the stays or something. If your bike is aluminum, on the other hand, then the regular should work fine.