We just noticed the same bad design on our back bumper.
I would be inclined to get a new full width piece of aluminum take it to a bender put an 's' drill the rivets from the outer skin along the bottom - slide up the new piece vulkum the *&^%. Remove the bumper lid and run the s over the bumper so that any water that gets in runs into the base of the bumper compartment instead of back towards the floor. Install a grate in the bumper compartment so that the water runs out freely - not drip holes as they will just get bunged up with dirt from the road.
Seems like an easy design in my head I wonder how easy it would be to fabricate and install