I have two plastic ones. For the smaller one I had to snip every other one of the little plastic things on the bottom so they could fit their bills in. The don't have a problem on the bigger one.
They look like this:
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I had that feeder for a while, but it was always getting clogged with food. So I switched to this kind of metal feeder and I love it.
For any ducks that I have to keep temporarily separate, I use a gallon jug with a hole cut in the top like in the picture below.