I’ve just given the thumb tack method a try. It’s very clever! It worked quite well - the beginning and end of my circle didn’t quite meet, but this was my first attempt. I had a problem with the thumb tack coming adrift because the masking tape I’d used tore, so I stuck duct tape on top of the masking tape (so that it was the latter in contact with the machine). That was fine. The only drawback was the small but noticeable hole in the fabric at the centre of the circle.