This is the hub collar. Before sewing it together, while still in a flat pattern, I ran a running stitch about 1/8"-3/16" from the pattern edge along the entire edge. I did the same thing on the hole of the "boomerang" piece as well. Then, I stitched by hand the collar ends together. Note how the stitching only goes part of the way up - this is because I left excess on the top of the collar.