Interesting . . . I cannot remember ever owning a car that let any significant amount of water in the trunk just from opening it. All my other cars successfully directed the water into the rain channels except for maybe a few drips. This car has streams of water coming off the trunk which totally miss the channel and go straight into the trunk.

 

I think the worst part of the trunk are the huge wells that hold water on the sides.
After washing the car, I have to open the trunk lid and soak up like 8 oz of water and suds from each well...where the hinges are, or it all flows out and wets my now dry and polished rear bumper when I drive away.

As for avoiding rain in my trunk?
I park in a garage!