This was the house we originally used for 3 Indian Runner ducks.
We purchased a dog house kit from Lowe's. Purchased some cedar boards to make a door along with a pair of hinges and a lock.
Completed house, with added light, door and lock
House complete as the dog house kit
Assembled according to directions
Close up of door
We used cedar tongue and groove boards to make the door and framed it out with another piece of cedar. Installed a thin piece on either side of the door opening on the dog house to attach hinges and the lock.
Back side of the door was painted with many coats of exterior paint for easy hose cleaning. The lower half and floor of the duck house was also painted with several coats of exterior paint before the ducks started using it. All I've done to clean it was use a hose and every now and then when I didn't hose daily and poop baked on, a scrub brush (but not often).
Room for 3 runners
I used a short plastic storage bin filled with leaves/pine straw for bedding. This is Blue who decided to try being broody for a few days sitting on 3 eggs... none fertile.
This dog house kit from Lowe's served us well. It worked fine for 3 adult runner duck girls. Once we decided to add a few more ducks, we upgraded and the Lowe's kit duck house is now in use for the new ducklings. We eventually added a eye bolt to the roof and lower portion of a wall to attach a bungee cord as added protection. Only thing I'd have done differently is to have made a way for water to drain out for daily cleaning (via hose) to go out without having to get it out the door. And paint the entire inside walls/floors instead of just half. The draining for cleaning wasn't a big issue, but would have made it quicker and a tad bit easier.
It is also a very portable house as it is light. Easy to move to new locations as needed.