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!