Plans for a small building show a 3- by 8-inch cardboard void form to be used around the exterior of the base of a grade beam wall. We don't know where to get such void forms. Is there any reason why we couldn't simply use a 3- by 8-inch strip of polystyrene foam?
This sounds like an excellent method of forming the void. Polystyrene foam is commonly used for boxouts and other forming necessities.