I have a Thule Evolution 1800 cargo box that I managed to grab for $50 off of craigslist. It's pretty big. The hatch does hit the cargobox when I have it mounted but I created a limiting strap with some paracord that prevents my hatch from opening all the way.

I personally wouldn't use the oem crossbars as they do sit too low. I wouldn't be able to use my cargo box on the oem crossbars because the box won't clear the shark fin. I've had two setups. My first setup was yakima cross bars on some yakima mounting adapters that attach to the stock roof rails. It works great and you usually can find a set of used crossbars on craigslist for cheap. You'll just need to buy the mounts separately. My second and current setup are the LFD crossbars. I prefer these as they sit higher than the oem crossbars and lower than the yakima setup. Perfect imo.