That's a good question with regard to store-bought ramps---one that I don't have an answer to. (I'm sure you'll get responses from those that do.) If you have any extra lumber laying around (or want to buy some), you can do what I did: I made ramps using 2 X 8 planks. (I did this a long time ago, for my Toyota---If I was doing it now, I'd use 2 X 10s.) You can choose the lengths so as to make a gradual "climb" so that everything on the Box will clear.
As you may have noticed, I later added little pieces of 1 X 8 to make ascending the ramps a little easier. (It worked pretty well without them, as well.)
