Is it that big of a deal to have the slats touch the skin. If you don't use slats, then your butt touches the skin and changes the shape anyway. I have put my seat slats on the underside of the frames with good luck (they are uncomfortable, but hopefully this summer I will come up with a better solution for that). Mine are 1/2x3 slats and they do bend. Assuming this would happen, I made them overlap the backside of the forms. Yours are much thicker and many more, so they might not bend.
How did you attach them to the frames anyway? And are you carving the underside of the seat to lower it?
Scott