So last year, I tried this project with my class and it was a disaster. A few kids got it but most of the projects were a hot mess. I told the kids to just start carving and to think about the overall shape they were leaving behind. They had to focus on the positive space, not the negative space that they were removing, even though those holes were going to be important. What I ended up with was lines and smiley faces carved into the foam...not exactly what I was looking for and not exactly focusing on the positive space.
After a lot of thought, I came up with a solution to the problem of freely carving. This is going to sound nuts but we scribbled! We took a sheet of paper that was long enough to wrap around the block of foam and scribbled...big loopy scribbles like this: