Have you looked at designs for dog houses? Ducks don't need nest boxes or perches, and they don't do well with ramps that are long or steep, from what I have read. I designed a basic big box using plywood and 2x4s. The roof was the most exotic part of it.
As it turned out, because the shelter is unheated, and some of my runners are not cold hardy, I ended up moving them into the walkout basement (big window, lots of space, very nice little area). So in addition to a nice looking, simple house, think about heating it when it's crazy cold, and how to keep it from getting dangerously hot when it's crazy hot.
My solution to avoiding watery mess in the house was to have a veranda - a porch - covered with half inch metal hardware cloth on top, bottom and sides. Water and food were kept out there, not in the house itself.
The house is tall enough for a foot and a half of deep litter, without the runners bumping their heads on the ceiling.