Thank you, Bob, Jeff!
Bob, Jeff has a point. I don't know that I could ever make a portal with the detail of this commercial foam one. However, I did watch a Joey Ricard video last evening of how to make a stone pier from a piece of extruded Styrofoam.
As Jeff mentioned foam rollers, I wondered what the one I am using for clay would do on foam. If I could exert more pressure, it wouldn't be too bad.