I bolted Oak 2x12s to the load bars on a set that we used in dirt/gravel alleys. It worked real well. Sometimes you had to put dirt or a rock under a corner to level but it worked. If the platform is sloped a little it will still work even with a couple of inches slope from end to end.
As muleskinner said, it doesn't have to be exact but need to be close. I set mine on 4 x 6 treated lumber buried to ground level. I used a 4' level on a straight 2 x 4 to level the bars. Used some 1/4 plywood squares for shims.