I found 1/4" foam was most useful on the first layer to fill in between the stamped in ridges. Once that's down you've got a single surface and you can just glue on a few more layers. Take your time sculpting, it's easier to take more off than put it back on. Sculpt a little, mount the seat, sit on it for a good few minutes, then sculpt some more. And dont be afraid to start over if it all goes to hell. I did at least once, maybe twice.
Scott