I have some of those breeds and inside my duck house I have some plastic tables which are actually sections from a big shelving unit some of the ducks jump up and sleep on top and some of them sleep under the clearance is 18 inches and they’re very happy with it, but don’t forget Ducks are very good at ducking.

Yes pun intended.
It doesn’t have to be so high that they’re standing completely upright underneath it. As long as they have other space to walk around without a shelf above them. I like to give my ducks at least 4 ft.² each floor space inside of their overnight coop.
Something funny I’m sitting outside with my Ducks now and the light breeds (my Indian runners) when they stand up right they’re taller than the big heavy Ducks lol they stand like a little bowling pins