I was going to buy one until someone here told me to just tape a thumb tack down on the machine's bed. Then you put on your fabric and you can stick an eraser on to the thumb tack to hold it in place if necessary. You can make any size circle you want - for the cost of a thumb tack..
This link shows just a straight stitch but you can do it with any stitch on your machine...
http://doodlequilts.blogspot.com/201...play-time.html