Put the egg whites in a large bowl, and sift in the icing sugar. Stir to make a thick, smooth icing. Spoon some of the icing into a piping bag with a medium plain nozzle. Pipe generous lines of icing along the wall edges, and join the walls together. Use small cans or bottles to support the walls, and then allow to dry, ideally for a few hours or over night in a dry environment.