To couple on what John wrote, I was just at Lowes yesterday buying dome wood for a shelf in my wife's laundry room. I went past the foam insulation and took notice it looked different from what I have seen. Dummy me, I should have taken a photo, because I knew you were planning to use 1/4 inch. I saw two types, both grayish green. One was 2 inch thick, the other was folded sheets of the 1/4 inch. Instead of having a smooth surface as I am used to, both had lots of random depressions like the foam doesn't fill completely in when it is being formed. For normal base or scenery work this would be fine, but the depressions are a bit deep for carving stone or block.
Like I said, I should have taken a photo; big dummy. Now I can't remember the name of it. It seemed that was all Lowes was selling except for the white bead stuff that isn't as useful around the layout. Has anyone seen this?
Think Lowes has some that is made to pass air thru. It is not a smooth surface like the standard pink and blue foam sheets.