Actually, with the Steripen's very narrow glass tube and rubberized base, you can insert the tube into a narrow mouth platy until it "stops" at the rubberized base. Then, you can turn the Platy upside down and agitate -- without a drop of water leaking out!
Sean first mentioned it up above, followed by a confirm from Chris Wallace. That got me pretty excited, so I called Miles @ Steripen to confirm effectiveness. Alas, negatore, so I wanted to pass this info back.