Tailgate rib delete/poor man’s Mountain Hatch

I think if you cut strips of high density foam to fill in the depressions, you would get the comfort without compromising strength/rigidity. Even if the foam was not perfectly level with the top, it would still be easier on your knees.

 

Yeah, mine are just routered halfway into the HDPE. The little ridges are enough to keep things from sliding if you're parked at an angle or something. I don't think I would do any more than that (like cutting holes or installing full sized cupholders).