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.