This thread is available in a variety of colors at Hobby Lobby. I have not found a comparable medium anywhere else. If you don’t have a Hobby Lobby nearby (God forbid!), Coats and Clark has a #18 Nylon Thread. Omega is stiffer and provides a more structured piece. Coats and Clark brand is a little ‘softer’, or more pliable. It’ll work if you keep your stitches tight.

Any decorative brooch, pin, patch, etc. can be the focal point on the cover flap of the case. Just make sure you pick something that is easy to attach by whipstitching, or by some other method. I look for embellishments that have spaces or holes around the edges that a needle can slip through easily.