slope on rear bumper

hello , instead of the rear bumper being flat on top could you built it so there is a bit of a slope to it ? that way in the winter the snow would run of it instead of sitting there freezing . when it freezes on there you cant get the hatch open unless you get your scrapper out and scrape the frozen snow off that's laying along the top of the rear bumper . also with the salt and snow laying up there the bottom of the hatch or the tailgate start to rust out before they should .. thankyou . peter vande velde .