I've communicated many times with Steripen tech support and the steripen handles a liter or a quart, and in fact, the steripen works just great even with a 1.9 oz Ziploc twist n Loc Container plastic quart container -- which has the advantage of much less weight and is 100% easy to clean out. Makes a great shake/drink container too (with the lid). I checked with Steripen about the okayness of such and it was okayed by Aaron in their tech support and I used it with a steripen for 23 days on the JMT this last summer. Just make sure the container is leak proof -- some of the ZipLock containers are not. You have to fill it up with some water, put lid on, and shake it to see if it is leakproof.
Here is what Steripen's support told me about this container (06/30/2009):
"The container shape of the Ziploc twist n Loc Container will not be a detriment to the effectiveness of the SteriPEN. When using your SteriPEN Adventurer, remember that you must agitate the water for effective treatment. To ensure the container shape does not adversely effect the treatment, simple stir the lamp of the SteriPEN Adventurer through the whole container.
My only concern with this container is solarization. Do you know what material is used to manufacture this container and its thickness? Certain plastics will yellow and become brittle when exposed to UV radiation. The container may crack after extensive use with the SteriPEN. This is the same as when you leave it outside in the sun for extended periods."
Links about container:
(you want the medium size)