I hope Nettie and Duckyfromoz check in here - they have so much experience with this topic.
I kept our runners in the house for three months due to the weather. I had thought we would have them out in their own house at six weeks or so.
It was actually kind of a treat!
In case it helps - I had them in a puppy playpen in our guest room. I lined the playpen with plastic poultry fence because they were so small when they were first put in there and could have gotten stuck and really hurt themselves with the 2 by 4 inch openings. Anyway, I placed two layers of 6 mil plastic on the floor, with the edges going up the sides of the playpen at least 6 inches, to catch splash.
I found that hardwood shavings were way too dusty, and pine shavings, well, I am allergic to pine shavings
So I bedded with old towels. It is some work, but the rinse water with their poop was the best stuff I ever put on my garden!
Then again, I have eleven ducks.
I used a three gallon waterer placed on top of upturned 5 ounce yogurt containers inside a large diameter, 4 inch deep rubber watering bucket (again to catch splash). For entertainment, I tied a piece of slender rope securely to the top edge of the playpen. I also gave them empty yogurt cups to play with.
Ventilation was a consideration - I usually left the door to the room open, and I have a wooden folding panel with slats that I placed in the doorway to keep the cats out but let the air flow through.
As long as I kept up with bedding changes, there was no strong odor - seriously. But I changed bedding at least three times a day.
How mobile is your duck? It may not need much space.