ya know i just did this in july aug this year on my leer shell. leer supplies shells with thule racks. they are lockable/removeable. i got their catalog and then scammed a yakima from rei.
after looking at what thule and yakima had to offer. i went yakima all the way. much more versatile.
these are becoming a "HOT" item to steal as it obvious some bucks are involved. so buy some locks.
i got the ginormous load warrior rack. and all the goodies. takes 2 guys about 2 minutes to put on and LOCK. and about 1 minute to remove.
if ya have a pick em up, think about buying a STEP that goes over rear wheel to stand up on to move stuff and tie down stuff. about $40 bucks more and worth it.