I strongly suggest you borrow or buy a good min-max thermometer and set it up exactly where you would keep the littles.
Place food and water in there, sprinkle the bedding to imitate the moisture that would be there when the ducklings are there. I found that a dry brooder I could easily keep at 90F, but once the ducklings were in there, splashing water, I could not bump it up above 80 until I got another heat lamp going.
The min-max will record how cold it gets over time. Some are digital, some not. If you check it and reset it daily at around the same time, you will know if you can keep the brooder at the proper temperature.
And this setup is predator-proof, right? I remember having a rabbit hutch I thought was safe. But it was not fastened securely, and a dog knocked it down the hill, it shattered, and the rabbit was killed.
Predators work hard to get at ducks. You probably know that, but I feel it's important to write in this thread for others.