You can keep them in a small container, like a water bottle, though the tank would be better. Frequent water changes are important in a small unfiltered container, though snails are far more tolerant than fish. Make sure you get out uneaten food and poop before they mold. Turkey basters are good for cleaning and changing that water in small containers. A wide container would be better than a tall skinny one, for gas exchange, and if you can put an airstone in it, so much the better. Something like a margarine tub, Cool Whip tub, or Tupperware container would be fine.