Barn door sloped to match! Commercial overhead doors you see in parking garages and such that meet sidewalks or city streets on a slope often have a tapered metal piece on the bottom of them to make the sealing edge match, but as mentioned curb appeal would be a casualty.
The Reelcraft L 70100 123 3 at $715 is stupid expensive, but certainly should last. I would link to it, but I cannot until I post 3 more times after this.
Adam