They do make some roughly 2.5" X about 6" interior overhead ceiling replacement light enclosures that seem to be sealed and possibly counld replace your cargo light fixture.  I doubt if they would be UV protected plastic though.  They are very bright and run very cool.  Lots of LED's in them and draw so little amps.  Could even use 2 or more of them mounted on nice looking light type bar using the OEM wiring.  
Of note:  Not to be used when not towing a light blocking trailer behind as there are laws prohibiting the use of bright white lights on the rear of vehicles.  The driver behind's vision is dangerously compromised.