I just use tin-foil; either dunked in star-san or sprayed with iodophor, depending on what's in front of me. Honestly, the stoppers or air-locks are really unnecessary for starters. Bacteria travels on dust particles. As
demonstrated by Louis Pasteur, there's no way for bacteria or other microbes to climb up and in to your container if it has some tin-foil over the top. Simple, cheap, and effective. Just don't seal off the container!
If I were to use a stopper, I'd probably clean and soak it in star-san before putting it on the flask and setting the flask in an ice bath. Either the star-san or the steam will sanitize it, but both would be almost fool-proof, and I'd be damned to neglect an extra line of defense!
+1 on the Fermcap! Though I sometimes get boil-overs even while using it!