I'm in two different Valentine pin swaps. Since I have all these zippers I guess I should use them! I tried to figure out how to sew the zipper on before I turned it but this is too small and the zipper didn't want to turn an inside curve. So I had to make the heart first, stuff it, and THEN hand sew the zipper to the edge. Came out pretty good though. A lot of handwork. I always put all my knots in the center where I make the slit for turning. It ends up pretty messy and puts a lot of strain on the fabric. You can kind of see that here.
So what I do is make a felt patch to cover it. This does two things, hides the mess and reinforces the center. I put glue on the inside of the patch, which also helps strengthen the threads, and then I whip stitch around the edge.
I always put my pin back with the opening/fastener at the bottom. That way if it opens while you are wearing it, it won't just fall off right away.