I have a 1930s house in which I am removing an interior wall to enlarge the kitchen. I am installing a header and arch to support the ceiling. The house has a curved cove between the walls and the ceiling throughout, with a radius of about 3". I read in an old TOH article about glueing molded styrofoam forms into the corner to create the curve and plastering over them. The problem is that I can't find these forms available anywhere. Where can I buy them?