I have found that the most critical part of sewing any circle or curve that has to fit together is to cut precisely. If the seam allowance on the convex and concave curves match, then your drunkards path or circle will fit perfectly. Virginia Walton sews circles into 'holes' without pinning! or with very few pins. The circle is cut exactly 1/4" wider than the seamline and the piece of fabric with the hole is cut precisely 1/4" smaller than the seamline. When it is clipped it cant be sewn exactly right. It sounds real simple, but it really does work - when I've messed up I can usually find the mistake is in the cutting.