There's one out there called KargoMaster (http://kargomaster.com
). I had one on my old rig. It rusted like hell, created a gap between the shell and the bed lip that was hard to seal. Also, it was heavey, and reduced performace noticably climbing the canyons. If you didn;t want it over the cab, you could leave that piece off during assembly.
I've since gone with a yakima mounted to a shell. Of course my shell is really bomber (http://www.vacationvillageslc.com/
)and the previous one wouldn't have been able to handle the job. I can't load 13 boats on at a time anymore, but that's not realistic these days.