The kennel works awesome. Of course you can only keep 2 - 4 ducks in there. I also give them the top half of one of those plastic dog houses for shelter.
The portion of my back yard where I keep the ducks is the healthiest spot of lawn we have. They eat the dandelions (they love them) and...
Here's my Black Swedish drake I got from Metzer's a couple years ago. I keep my ducks in a dog kennel covered in chicken wire. I pull it to a new spot in the yard every day or so, that way there's no pen to clean!