A friend who has done two beautiful cushion covers has asked me about this. She wants to have them stretched straight. As you know, the stitching pulls the canvas into a parallelogram. Does anyone know where she might send it away to be straightened?
If your friend had used specialist tapestry canvas it shouldn't have distorted. As that type of canvas is very stiff . Is she going to make them into cushion covers if so is she going to see a pre zipped cushion back. Framers have specialist equipment they use to pull linen and Aida fabric back straight before framing. Perhaps they could straighten it for her.