Thanks Jules -
I saw that one, but the problem with their list in this instance is that it didn’t specify the extruded or the expanded 
The Blue-board is extruded, and the old styrofoam pellet coolers are the expanded. And I’ve read other sites advertising that their thinner Polystyrene is ‘perfect for laser cutting’ so I’m bemused. 
I may bite the bullet and try a tiny cut just to see what happens… I know we’ve got scrap blueboard at the theatre. 
That said - anyone have any alternative suggestions for materials that would substitute? Light weight, and thick enough to give dimension on stage?