One way to make the "ground glass" is to obtain a piece of glass (or clear plastic) the correct size, then instead of using abrasive to produce the diffuse surface apply "magic tape" self adhesive material in strips across the glass, taking care not to overlap anywhere, and also to cover the edges where the glass makes contact with the camera film gate so that the diffuse surface is at the same location as the film emulsion. It helps to use a brightly illuminated "brick wall" and arranging an improvised shade around the back of the camera.