lol, i've officially become the indoor duck person. lol. I have an indoor pen i just made this week.
I've have my ducks inside since they hatched in august. I originally got them when i lived in chicago, but i soon moved to alabama after i got them. My townhouse society doesn't mind me having them but i can't keep them outside.
If you don't already have the ducks inside, research more! I highly reccommend the book, "Duck there's a goose in the house!" by Nancy townsend (you can buy it here: thegoosesmother.com). It is the only book on indoor waterfowl and it has TONS of essential info.
I also reccommend you spend the time making a pen with a lot of features... I promise you'll thank yourself later. I made mine 4x9, a foot off the ground, and 2 feet high. There's a nest box with a lid at the end where they lay their eggs. I cut holes in the floor and rigged two kitty litter pans under the holes. Between the holes and the pans i put a 1/2inch wire. I put their food and water on it seperately to help with the mess they make when eating. I just posted pics of the pen here:
https://www.backyardchickens.com/forum/viewtopic.php?id=131278
I'll also post my hand drawn plans on there after this post. Hopefully it'll give you some ideas.
Be forwarned, once a duck gets used to the comfort of 70degree stable temps... they get spoiled. Mine will literally walk outside, feel if it's too hot or cold and immediately walk back in and sit under the vent. lol. they are brats.
Also, if you raise them inside from babies and plan on keeping them inside all of the time, IT IS A TON OF WORK! It's like having 2 year old children in your house. They get into everything, you constantly have to clean up after them and for them, they throw tantrums, and they will expect to be treating like humans... Mine get offended when they arent allow to sit at the dinner table or go in my kitchen (which we're working on, lol). I literally have up baby gates to keep them out of parts of my house.
If you don't build a huge pen, i highly reccomend you bring your ducks outside once a day and get them diaper harnesses so they can walk around and get exercise in the house. They will get restless in the pen and they will quack to be let out. Also, ducks raised strickly indoors will always want to be in your presence, hence why the diapers are very convienent.
If you have any specific questions or anything at all, feel free to pm me! I'm probably the expert on keeping ducks inside... lol. I've figured most of it out the hard way too!