Here is a coop I found when I was building my chicken coop that you may like! It's low to the ground so probably better for ducks anyhow
Of course, you do need to buy the plans but honestly you really probably don't unless you want/need more specifics! They have 4 good pictures of it and you can essentially see how to build it from the pictures without buying plans
https://www.meyerhatchery.com/productinfo.a5w?prodID=NECP
But also 4x8 is a pretty easy size to build, that's how big my coop is that we're building, and we found a really cheap plan on here but we did modify it a bunch haha they used OSB/particle board which would just fall apart so we used 3/4 pressure treated plywood for the floor and pine sheathing for the walls and I think roof, I forget what thickness. But we also covered it with tar paper and cement clapboard we found cheap on Craigslist and then shingles on the roof. If you're not going to cover it with some sort of siding or paint/protectant then you would really probably want to go with regular plywood.
But a 4x8 coop is pretty easy, you just need a bunch of 2x4s, plywood, and nails and/or screws essentially.
But we built ours on a 6x8 frame with pressure treated 2x6s so that we could add a storage area and nest boxes on the extra 2 feet. The nest boxes I think are 2x4 feet and we made 3 boxes. We were going to do 12x12 inches (1 foot by 1 foot) but I think they're bigger, I forget what we decided on haha I think 15 or something like that. 1 foot is plenty big enough for chickens but not sure about SAs. You would probably want to go like I did, a bit bigger. The bump out makes it really easy to collect eggs and most birds use it. I don't think ducks really fly much? And like to nest on the ground from what I've seen. So if you put the nest boxes indoors they would have to be on the floor or low to the ground which would mean the eggs might get dirtier and you would also need to bend down a lot which I don't think you would want to do. A bump out would allow the nest to still be on the ground but make them much much easier to collect.
We didn't end up building the storage area because we put a window in the wall haha but I do think a storage area is a good idea, next coop.
And another thing, I would maybe make the coop bigger than you think you need in case you do add more birds later.
My dad insisted on only 4x8 because it's a "mansion"/chicken palace compared to their current coop and he didn't want anymore chickens (we have 8) but I really wish we had just gone 6x8 or even 8x8. We already built it on a bigger frame anyway so it really wouldn't have cost anymore, except maybe a couple more pieces of wood. Now I am wanting a few more chickens and there's no room in the coop.
So definitely go a bit bigger if you can.
But 4x8 can hold up to 8 chickens (or ducks) at 4 sq. ft. each so there should be plenty of room.