You should try to find a bigger container. It could be almost anything that you can sanitize. I bought a 1 gal glass jar of pickles in a pinch once, it was like $5 and I got a bunch of pickles too. Even a 2 liter Coke container or something would work. It's better to make a starter of at least 1 qt. with a reasonably fresh vial of yeast. Check out http://Mr Malty's <i>Fourteen Essent...t Starters</i>
. Also use the pitch rate calculator from the same site to see exactly how big it should be.
You can just use a piece of aluminum foil, no need for an airlock. The foil is actually a little better because it allows a small exchange of air (you want this in a starter but not in a regular beer fermentation).
I would start it either tonight or Thursday. You never know when the yeast will be slow to start and you want to allow enough time for it to finish and completely settle
. FWIW, I usually do starters for Saturday on Thursday night.