here's a very elaborate idea that may be too much of a pain to use... get a 20 oz soda bottle, and poke holes in it big enough for snails to enter, but too small for shrimp. Inside the bottle, place cucumber AND... a little puffer.
small snails can crawl to the top of the glass, flip upside down, and "crawl" on the underside of the surface of the water if there is surface tension. it's still probably better to sink it though...