...now you don't!
When I built Igelfeld I found it was hard to carve into the Styrofoam after it had been installed since access with the knife was tricky. To get around this, I cut out these chunks before the sheet was laminated to the base. The idea is to carve them on the workbench and use the flat sides of the cuts to locate them when they're ready to be installed.
Taken: Sunday 11th October 2009