I really swear by using a Cool Whip Tub with an oval slice cut from the lid. Don't make the oval too large, or succumb to the idea two openings would be better; it won't. You need the grip of the lid to keep the lid on the container. Two holes weaken that hold, and ducklings getting on top of the tub will get their feet stuck in it.
I use the great big container, with a clean roundish, flat-ish rock to weigh it down.
I use the great big container, with a clean roundish, flat-ish rock to weigh it down.