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 provide natural fertilizer. I don't have to clean a pen and they always have a clean, fresh environment.